- Email: [email protected]
Frequency stable I2 Raman laser excited by a cw frequency doubled monolithic Nd:YAG laser
1 October 2000
Optics Communications 184 (2000) 215±223
www.elsevier.com/locate/optcom
Frequency stable I2 Raman laser excited by a cw frequency doubled monolithic Nd:YAG laser M. Klug a, K. Schulze a, U. Hinze a, A. Apolonskii b, E. Tiemann a, B. Wellegehausen a,* a
b
Institut f ur Quantenoptik, Universit at Hannover, Welfengarten 1, D-30167 Hannover, Germany Institute of Automation and Electrometry, University of Novosibirsk, Novosibirsk 630090, Russian Federation Received 26 April 2000; received in revised form 27 July 2000; accepted 1 August 2000
Abstract Continuous Raman laser operation with 127 I2 , 127 I129 I and 129 I2 molecules, excited by a frequency doubled diode pumped monolithic Nd:YAG ring laser is reported. Within the tuning range of the pump laser, the molecules possess several absorption transitions, which so far enabled laser operation on more than 50 lines in the spectral range of 544± 1330 nm. Multi-line and single-line operation in standing wave and ring resonators have been investigated. With pump powers of up to 500 mW, output powers up to 10 mW on individual lines were possible. Due to the narrow Raman gain pro®les (down to 100 kHz) and the narrow pump laser line width of 10 kHz of the Nd:YAG, correspondingly narrow Raman laser line widths are expected. Therefore, with the pump laser stabilized on a speci®c hyper®ne iodine absorption transition and a frequency locking of the Raman laser resonator to the induced Raman gain pro®le, a system with many reference laser lines and with expected frequency stabilities of dm=m 10ÿ10 can be realized. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 42.65.k; 42.55.Y; 33.55.B Keywords: Nonlinear optics; Frequency stable lasers; Raman lasers; Dynamic Stark eect
1. Introduction Raman type three level laser systems have already been investigated both experimentally and theoretically [1], and cw operation with various diatomic molecules has been demonstrated [2]. Refs. [1,2] give corresponding reviews. Due to the simplicity of the laser scheme, the good conversion eciencies achievable and the easy single fre*
Corresponding author. Fax: +49-511-762-2211. E-mail address: [email protected] (B. Wellegehausen).
quency operation on many lines covering broad spectral ranges, the systems have potential for applications in spectroscopy and metrology. Especially, molecular iodine is an interesting Raman material, because it has a narrow Raman line width [3], and can be pumped with available powerful pump lasers in the green spectral range. It has a dense emission spectrum throughout the visible and near infrared spectral range [4], and can easily be generated at room temperature in sealed-o quartz or glass cells. Furthermore, molecular iodine which is spectroscopically well characterized [4±7] and used for absolute frequency
0030-4018/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 ( 0 0 ) 0 0 9 2 1 - 4
216
M. Klug et al. / Optics Communications 184 (2000) 215±223
stabilization of lasers, and several transitions are recommended as wavelength standard [8,9]. Spectroscopic investigations in the tuning range of the frequency doubled Nd:YAG laser have been done with pulsed [10] and cw [11] lasers. Raman lasers in molecular iodine have been operated with dierent pump lasers, pulsed dye and pulsed frequency doubled Nd:YAG lasers [12], the 514 nm line of the cw argon-ion laser [13,14] and even the low power He±Ne laser line at 633 nm [15]. Instead of using molecular iodine in a cell, cw operation with extremely low threshold was also achieved with a molecular beam as Raman material [16]. In order to use the interesting spectral features of iodine Raman lasers for applications, it seems necessary to replace the dye or argon laser pump source by a more compact and reliable pump source, which in our opinion can be a cw diode pumped frequency doubled Nd:YAG laser. Such lasers are now available with high output powers, and when starting from a monolithic Nd:YAG ring laser, narrow-bandwidth single frequency operation with line widths below 10 kHz is guaranteed [17]. Frequency doubled diode laser pumped monolithic Nd:YAG laser systems have successfully been used for pumping of cw optical parametric oscillators and the generation of narrowband tunable radiation [18]. In this paper, we report cw Raman laser operation in 127 I2 , 129 I2 , 127 I129 I molecular iodine pumped by such an all solid state laser system. Investigations on the absorption transitions, on the laser operation in standing wave and ring resonators and on the line width and frequency stability are presented, and potential applications of the Raman laser system will be discussed.
the temperature of a sidearm. The temperature could be stabilized and varied between 0°C and 40°C. The temperature of the tube was kept slightly higher than that of the sidearm temperature in order to prevent condensation of iodine vapor on the Brewster windows. The experimental setup is shown in Fig. 1. Stokes±Raman lasers have been investigated using (a) simple two mirror standing wave resonators and (b) folded four mirror ring resonators. In the case of the standing wave resonator, the pump radiation is focussed into the resonator through mirror M1, which has a transmission of 90% for the pump radiation and a maximum re¯ection for the Raman laser lines. Frequency selection is realized by inserting a birefringent ®lter (BF) or a high transmittive interference ®lter into the resonator. The generated laser radiation is coupled out via mirror M2 or by an additional quartz plate (QP), which is inserted into the resonator at an angle close to the Brewster angle. The ring resonator consists of two spherical mirrors (M1, M2) and two ¯at mirrors (M3, M4), and a BF as frequency selective element. Mirror
2. Experimental For the laser investigations, dierent types of sealed-o Brewster windowed quartz cells were used. Two tubes with a length of 165 or 185 mm and a diameter of 25 mm were ®lled with spectroscopic grade 127 I2 iodine. Two other laser tubes (length: 250 or 500 mm and diameter: 6 mm) contained mainly 129 I2 and a small amount of 127 I2 , which led to the formation of the mixed 127 I129 I molecule. The vapor pressure was controlled by
Fig. 1. Experimental setup for generating Raman lasers in I2 (a) standing wave resonator and (b) folded ring resonator. M1± M4: mirrors, BF: birefringent ®lter, QP: quartz plate for output coupling, L: focussing lens, F: ®lter for blocking pump radiation, D: photodiode, PZT: piezoelectric transducer, HV: highvoltage supply, NF: low frequency generator, and TK: thermo controller.
M. Klug et al. / Optics Communications 184 (2000) 215±223
M3 is mounted on a low-voltage piezoelectric transducer (PZT) for ®ne adjustment of the cavity length and stabilization of the Raman laser frequency via lock-in technique to the maximum of the gain pro®le.The pump radiation passes mirror M4 (transmission for the pump radiation about 90%) and is focussed by M2 into the vapor cell. All mirrors are highly re¯ective for the generated Raman laser radiation. M1, M2 and M3 are also highly re¯ective for the pump radiation. Output coupling of the generated Raman radiation is done by inserting a QP into the resonator. According to the much stronger forward ampli®cation [1,2], the Raman ring laser oscillates only in the direction of the pump radiation. The standing wave resonator was mainly used for the evaluation of possible pump and Raman laser lines, while the ring resonator was applied for all investigations on frequency stable single line operation.
3. Laser lines and laser operation For a proper identi®cation of possible pump transitions within the tuning range of the pump laser, absorption spectra have been taken for both types of cells simultaneously. Using the wellknown 127 I2 absorption spectrum of the B±X system (level scheme in Fig. 2) as reference [5], the absorption spectra of 129 I2 and 127 I129 I can easily be
217
assigned. The pump laser can be tuned by the temperature of the Nd:YAG laser crystal with a tuning coecient of approximately 8 GHz Kÿ1 ; mode hops occur every 2 K. Within the tuning range of the pump laser (18 787.1±18 789.5 cmÿ1 ) several strong transitions from v00 0 in the ground to v0 32±37 in the B state of the three isotopomeres have been found. The identi®cation of the resonances in the 127 I2 is straightforward using the very well-known molecular parameters [19] or the iodine atlas [5] and is given in Table 1. The identi®cation of the pump transitions in the cell with the mixed isotopomeres is based on the Dunham parameters for 127 I2 , which have been recalculated for 129 I2 and 127 I129 I according to the dierent molecular masses [20]. Assignments can be found in Tables 2 and 3. Table 1 Pump transitions in the tuning range of the frequency doubled Nd:YAG laser in 127 I2 (the line number corresponds to the numbering given in Ref. [5])a Line
Transition
FCF 103
(cmÿ1 )
1106 1107 1108 1109 1110 1111 1112 1113
P119(35-0) R86(33-0) R106(34-0) P83(33-0) R56(32-0) P53(32-0) P142(37-0) R121(35-0)
28.5 30.6 29.6 30.6 31.4 31.4 25.9 28.5
18 787.12 18 787.27 18 787.34 18 787.80 18 788.33 18 788.41 18 788.79 18 789.17
a On transitions indicated in bold, laser operation was investigated.
Table 2 Pump transitions in the tuning range of the frequency doubled Nd:YAG laser in 129 I2 a
Fig. 2. Relevant potential energy curves: within the tuning range of the frequency doubled Nd:YAG laser at 532 nm, pump transitions between v00 0 and v0 32±37 have been identi®ed. Raman laser lines in the range v00 2±72 have been operated.
Transition
FCF 103
(cmÿ1 )
R143(37-0) P115(35-0) R102(34-0) R46(32-0) P43(32-0) R80(33-0) P99(34-0) P77(33-0) R45(32-0) P42(32-0)
26.1 28.7 29.8 31.4 31.4 29.7 28.9 29.7 30.3 30.3
18 787.21 18 787.66 18 788.00 18 788.13 18 788.17 18 788.64 18 788.93 18 789.03 18 789.45 18 789.49
a On transitions indicated in bold, laser operation was investigated.
218
M. Klug et al. / Optics Communications 184 (2000) 215±223
Table 3 Pump transitions in the tuning range of the frequency doubled Nd:YAG laser in 129 I127 Ia Transition
FCF 103
(cmÿ1 )
R52(32-0) P49(32-0) R133(36-0) R120(35-0) R104(34-0) R83(33-0) P80(33-0) R51(32-0) P48(32-0) P101(34-0) P117(35-0)
31.4 31.4 27.4 28.6 29.7 30.7 30.7 31.4 31.4 29.7 28.6
18 787.21 18 787.28 18 787.45 18 787.72 18 787.82 18 788.21 18 788.67 18 788.72 18 788.79 18 788.84 18 789.42
a On transitions indicated in bold, laser operation was investigated.
Strong absorption transitions where laser operation has been investigated are indicated in bold, but all other transitions should also be applicable for laser purposes. The threshold for Raman laser operation is dependent on the losses of the resonator, the cell length, and the vapor pressure. Here, typical pump thresholds around 60 mW in ring resonators were achieved. Above the threshold, the output power ®rst increases linearly with the pump power and then saturates. To achieve maximum output power, the vapor pressure and the output coupling have to be optimized. For strong lines, maximum output powers of about 10 mW at pump powers up to 500 mW were observed so far. The output power for 129 I2 laser lines and mixed isotopomeres lines was typically higher than that for 127 I2 , probably due to the longer cells. In the case of the mixed isotopomere, the vapor pressure seems to be too low to achieve optimum laser operation. From output coupling measurements, gain values of 0.002 cmÿ1 were determined for lasing at wavelengths of 599, 604 and 618 nm in 129 I2 at pump intensities of 500 W cmÿ2 (230 mW at a focal diameter of 240 lm of the pump laser). For a resonator without frequency selection elements, oscillation may occur simultaneously on several lines, depending on the mirror characteristics. However, as all lines share the same upper laser level and as the corresponding narrow Raman gain pro®les are not likely to be simulta-
neously resonant with dierent modes of the resonator, this kind of operation is quite unstable and leads to strong intensity ¯uctuations for the lines. Stable operation is only achieved if a single line is selected by means of a highly selective birefringent or interference ®lter and if the resonator length is stabilized to the gain pro®le. This stabilization and complications, which result from the hyper®ne structure, will be discussed below. With the given pump laser at 532 nm and the corresponding absorption transitions of Tables 1±3, so far Raman laser lines have been operated and spectroscopically identi®ed in the spectral range of 544±1330 nm (Tables 4±6). For measuring the laser lines, a 0.5 m monochromator is used with an accuracy of 0.5 nm. The P and R branches can be resolved over the whole detected spectral region. The values presented in Tables 4±6 are calculated values. To our knowledge, laser lines from the mixed 127 I129 I isotopomere have been observed for Table 4 Realized Raman lasers pumped by R56(32-0) in quencies calculated from Ref. [6]) v00
Frequency (cmÿ1 ) R(56)
P(58)
31 33 35 38 40 42 44 47 49 51 53 54 55 56 57 58 60 62 64 66 67 68 69 71 72
12 803.62 12 465.82 12 134.68 11 650.90 11 337.29 11 031.08 10 732.51 10 299.55 10 021.25 9751.58 9490.86 9363.95 9239.41 9117.28 8997.60 8880.42 8653.72 8437.54 8232.23 8038.10 7945.33 7855.47 7768.53 7603.57 7525.57
12 795.96 12 458.23 12 127.16 11 643.49 11 329.96 11 023.83 10 725.35 10 292.52 10 014.31 9744.74 9484.12 9357.27 9232.78 9110.71 8991.08 8873.96 8647.38 8431.33 8226.15 8032.17 7939.47 7849.69 7762.83 7598.03 7520.12
127
I2 (fre-
FCF 103 7.8 8.8 7.9 5.5 8.6 10.2 9.6 6.2 10.3 13.1 13.5 2.1 11.5 6.7 7.7 12.8 19.2 24.5 27.0 24.9 24.2 14.7 64.2 130.0 210.0
M. Klug et al. / Optics Communications 184 (2000) 215±223 Table 5 Realized Raman lasers pumped by P77(33-0) in cies calculated from Ref. [7]) v00 2 4 5 6 7 9 11 13 15 17 22 24 26 28 31 33 35 37 38 39 40 42 44 46 48
Frequency (cmÿ1 ) R(75)
P(77)
18 379.45 17 963.50 17 757.39 17 552.52 17 348.90 16 945.46 16 547.13 16 154.01 15 766.18 15 383.72 14 451.75 14 088.92 13 731.97 13 381.03 12 866.19 12 530.93 12 202.22 11 880.26 11 721.87 11 565.25 11 410.40 11 106.22 10 809.54 10 520.62 10 239.75
18 368.08 17 952.42 17 746.34 17 541.51 17 337.92 16 934.55 16 536.30 16 143.25 15 755.49 15 373.11 14 441.33 14 078.59 13 721.72 13 370.86 12 856.16 12 520.99 12 192.37 11 870.51 11 712.17 11 555.60 11 400.80 11 096.72 10 800.15 10 511.35 10 230.59
129
I2 (frequen-
FCF 103 13.1 7.8 5.5 4.5 7.0 8.2 8.7 8.4 7.3 5.5 7.2 7.9 7.3 5.6 6.1 8.0 8.5 7.5 2.5 5.2 5.4 8.3 10.0 10.0 8.2
the ®rst time. From the potential curve of the iodine molecule (Fig. 2), laser oscillation should be possible up to wavelengths of about 1.5 lm. Infrared laser lines up to 1.35 lm v00 72 have been observed. Longer wavelengths could not be detected so far due to limitations of our monochromator and detector system. For determination of the observed Raman transitions to v00 > 19 in the ground state, the data set from Bacis and coworkers [4,6,7] is used for frequency calculation.
4. Single line operation and line width of free running Raman lasers To achieve single frequency operation of the iodine Raman laser, in addition to the selection of a single rovibronic transition as discussed above, a single hyper®ne component on the pump and Raman transition has to be selected. The pump
219
Table 6 Realized Raman lasers pumped by P80(33-0) in quencies calculated from Ref. [7]) v00 4 6 7 9 11 13 15 17 18 22 24 26 28 33 35 37 39 42 43 44 46
Frequency (cmÿ1 ) R(78)
P(80)
17 960.67 17 548.24 17 343.92 16 939.09 16 539.43 16 145.01 15 755.95 15 372.28 15 182.52 14 437.54 14 073.69 13 715.77 13 363.92 19 135.08 19 135.08 19 135.08 11 544.31 11 084.62 10 935.15 10 787.62 10 498.47
17 949.06 17 536.71 17 332.42 16 927.67 16 528.08 16 133.75 15 744.76 15 361.18 15 171.45 14 426.63 14 062.87 13 705.05 13 353.28 19 135.08 19 135.08 19 135.08 11 534.21 11 074.69 10 925.28 10 777.80 10 488.77
127 129
I
I (fre-
FCF 103 8.8 5.5 6.0 7.4 8.3 8.5 7.8 6.4 2.5 6.5 7.7 7.8 6.6 7.2 8.5 8.3 6.6 7.0 1.5 9.5 10.5
laser can be stabilized on hyper®ne components by means of FM saturation spectroscopy [11]. The selected hyper®ne pump transition then leads to gain on only one speci®c hyper®ne component of the Raman transition, according to the selection rule DF DJ for J P 20. The situation for the transition considered below for the line width investigations is explained in Fig. 3. Within the Doppler pro®le of the selected hyper®ne component (about 270 MHz at T 300 K), other hyper®ne components may also be excited o-resonance, which may lead to o-resonance oscillation on additional hyper®ne components. This situation can be improved by selecting isolated hyper®ne pump transitions (e.g. a1 , a10 , a15 ). Comparable isolated hyper®ne components can be found for all transitions with high even J values, but R56(32-0) is especially favorable for us. For investigations of the line width and frequency stability of the Raman laser, the Raman transition R56(31-32) at 12 803.6 cmÿ1 was chosen. For this Raman transition, the Franck±Condon
220
M. Klug et al. / Optics Communications 184 (2000) 215±223
Fig. 4. Shape of the output power pro®les at dierent pump intensities: (a) 260 W cmÿ2 , (b) 3 kW cmÿ2 , and (c) 4 kW cmÿ2 . The threshold pump intensity is less than 200 W cmÿ2 .
Fig. 3. Example of pumping a single hyper®ne line (e.g. R56(320) a10 [11]) by a frequency doubled Nd:YAG laser solid line of transition j1i±j3i. Adjacent lines (dotted) are D 150 MHz apart and are pumped o-resonant within the indicated Doppler pro®le. On the Raman transition j3i±j2i, the induced narrow forward gain pro®les [2,21] are clearly separated and the central component (solid line) is favored (D0 120 MHz) for R56(31-32).
factor (FCF) has a local maximum; standard photodiodes are most sensitive and a good set of mirrors was available. To investigate the line width and frequency stability of the Raman laser, two almost identical ring laser systems (Fig. 1b) have been built in order to allow beat frequency measurements. As the Raman gain pro®le can be very narrow (less than 80 kHz at vapor pressures of 0.07 Pa and pump intensities of 5 mW cmÿ2 [21]), the length of the resonator has to be controlled and stabilized in order to keep one mode of the resonator within the pro®le (mode spacing 150 MHz). Single frequency operation can thus be controlled by using the resonator itself as a scanning Fabry±Perot interferometer. In addition, by scanning the resonator length and measuring the output power of the Raman laser, an output power frequency pro®le is obtained from which a rough estimation of the width of the Raman gain pro®le may be obtained (see discussion below). Such measurements are shown
in Fig. 4 for dierent pump intensities. At low pump intensity (close to threshold), a narrow single peak pro®le with a width of about 4.1 MHz (FWHM) is obtained, while at higher pump intensities, the pro®les are split due to the dynamic Stark eect. For the line width and frequency measurement, the intensity was kept low, in order to avoid the splitted pro®le. For locking one mode of the Raman laser resonator to the induced hyper®ne power pro®le, the position of M3 is modulated by a PZT with a frequency of 1 kHz (Fig. 1b). The corresponding frequency modulation depth has to be smaller than the width of the gain pro®le. The resulting amplitude modulation of the output power is then detected by photodiode D and demodulated by a lock-in, which corrects the oset of the PZT to ®x the laser to the maximum of the gain pro®le. In this way, amplitude stabilities of the laser system of better than 10% over minutes are reached. Without the locking, the laser switches on and o within time intervals of a few seconds, indicating strong frequency drifts and frequency ¯uctuations. To measure the laser line width, parts of the laser emission (100 lW) of both lasers are overlapped on a beamsplitter and focussed onto an avalanche photodiode. The detected RF spectrum is analyzed by a spectrum analyzer (HP4395). An example of the RF spectrum is shown in Fig. 5. The spectrum shows a width of the beat signal of 4.5 kHz (sweep time: 220 ms) from which a contribution of each Raman laser of 3.2 kHz may be deduced assuming statistically independent la-
M. Klug et al. / Optics Communications 184 (2000) 215±223
Fig. 5. Measured beat spectrum of the two Raman lasers: the resolution bandwidth is 1 kHz and the sweep time is 220 ms.
sers. This line width is smaller than the free running line width of about 10 kHz in 100 ms of the pump laser. For the interpretation of this result, a brief discussion of factors in¯uencing the Raman gain pro®le and with it the power pro®le and the Raman laser frequency stability are helpful. The small signal Raman gain pro®le at resonance conditions in general has a complicated structure and a forward±backward asymmetry due to the superposition of single- and two-photon contributions. For a more detailed discussion of the gain pro®les see Refs. [1,2]. For low pump intensities (no power broadening and no dynamic Stark splitting) and considering the forward direction, a nearly Lorentzian type pro®le may be obtained, which is assumed here for simplicity, with a center frequency x0R x32 kP =kR x0P ÿ x31 and a spectral width (FWHM) DXR of kP kP Dx23 ; DXR Dx12 1 ÿ kR kR where x31 and x32 are the transition frequencies on the 3±1 pump transition and on the 3±2 Raman transition. x0P is the pump frequency; Dx23 and Dx12 are the homogenous pressure broadened line width of the single photon transition 3±2 and the two-photon transition 2±1, respectively, and 1 ÿ kP =kR is the Doppler compensation factor with kP and kR being the pump and Raman laser wavelengths. In the case of iodine, the corre-
221
sponding values can be calculated using the relaxation rate and collision cross-sections given in Refs. [22,23], and yield for the above considered transition, Dx12 2:0 MHz and Dx32 4:1 MHz (at p 27 Pa) and DXR 2:6 MHz. Experimentally (Fig. 4) power pro®les were measured, which only indirectly re¯ect the gain pro®les, as under lasing conditions saturation and propagation effects have to be considered. The width of 4.1 MHz of the power pro®le at low pump intensity (Fig. 4a) best approaches the above situation and is in good agreement with the calculated value of 2.6 MHz. A ¯uctuation of the pump frequency around x0P x0P x31 will move the pro®le and broaden it when averaged in time. If the ¯uctuating frequency of the mode of the resonator is ideally (i.e. instantaneously or the bandwidth of the stabilization circuit is suciently wide) locked onto the center frequency, x0R , ¯uctuations of the Raman resonator will be fully removed, and the Raman laser frequency will follow the pump frequency with the reduction factor kP =kR and consequently will have a corresponding line width. From the 10 kHz width of the pump laser, we can therefore expect at best a spectral width of one Raman laser of 532 nm=781 nm 10 kHz 7 kHz under these conditions. For a non-instantaneous locking, only part of the mode ¯uctuations can be removed and only part of the pump frequency ¯uctuations can be followed. Consequently, the resulting Raman laser line width will then contain contributions from the pump laser line width and from the Raman laser resonator itself in addition to the contribution from the frequency modulation. In the described beat frequency experiment, both Raman lasers are pumped by the same pump laser, which means that they are no more statistically independent. It is expected that the Raman lasers will have correlated frequency ¯uctuations resulting from the pump laser and stabilization circuit, which to a ®rst approximation will disappear in the beat spectrum. This is also indicated by the fact that for these experiments, a free running (drifting) pump laser could be used, without any in¯uence on the beat frequency spectrum. The measured line width of the beat signal therefore mainly represents the line width contribution due to the short-term ¯uctuations from the Raman
222
M. Klug et al. / Optics Communications 184 (2000) 215±223
laser setup itself. These may be further reduced by a more stable setup and with a larger bandwidth of the stabilization circuit. In order to measure the real Raman laser line width (including contributions of the pump laser) either beat measurements with independently pumped Raman lasers or direct measurements with a high ®nesse cavity have to be performed. Such experiments are in progress. The present investigations, however, already show that narrow line width Raman lasers are possible. Therefore, with a stable pump laser locked to an iodine hyper®ne transition, correspondingly stable Raman lasers may be realized for all transitions having FCF above 5 10ÿ3 (Tables 4±6). 5. Conclusions In this paper, investigations on iodine Raman lasers pumped by a stable narrowband frequency doubled Nd:YAG laser are presented. Within the tuning range of the pump laser, at least 29 pump transitions for 127 I2 , 127 I129 I and 129 I2 isotopomeres exist, which allow Raman laser oscillation. Raman laser lines could be realized in the spectral range of 544±1330 nm, for the ®rst time also in the 127 I129 I isotopomere. All pump and laser transitions could be assigned, and based on molecular data, wave numbers for all involved transitions were calculated with a high accuracy. So far, performed power pro®le and line width measurements demonstrate that narrow line width (kHz) Raman lasers in iodine can be operated. With the present pump laser system, stabilized to an iodine hyper®ne transition, it is possible to operate Raman lasers with a relative frequency stability of dm=m 10ÿ10 for all transitions having FCF P 5 10ÿ3 . Concerning the absolute stability, further long-term stability investigations are necessary, and especially the role of the dynamic Stark splitting has to be studied. Pump and Raman lasers perform a two-photon transition, which leads to a coupling of the ®eld frequencies. The dierence frequency of these coupled (bichromatic) ®elds may have a much smaller line width as the incident ®elds [24], which is of interest in metrology and high precision spectroscopy.
Bichromatic ®elds with small dierence frequencies (GHz range) have successfully been applied in high precision hyper®ne measurements [25] and in preparing dark resonances [26]. Raman coupled bichromatic ®elds generated in the Na2 molecule have recently been applied by us for ef®cient dierence frequency mixing [27] and demonstration of parametric ampli®cation. By the Raman process in iodine stable dierence frequency ®elds in a wide range of dierence frequencies may be generated. Investigations on line width and frequency stability applying a direct representation of the bichromatic ®eld in a nonlinear crystal are in progress.
Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft within the SFB 407.
References [1] I.M. Beterov, V.P. Chebotaev, Progress in Quantum Electronics, vol. 3, Part 1, Pergamon Press, New York, 1974, p. 1. [2] B. Wellegehausen, IEEE J. Quant. Electron. QE15 (1979) 1108±1130. [3] J.B. Koend, S. Goldstein, R. Bacis, R.W. Field, S. Ezekiel, PRL 41 (1978) 1040±1043. [4] R. Bacis, D. Cerny, F. Martin, J. Mol. Spectrosc. 118 (1986) 434±447. [5] S. Gerstenkorn, P. Luc, Atlas du Spectre d'Absorption de la Molecule d'Iode, Laboratoire Aime Cotton, CNRS II, 91405 Orsay, France, 14 000±15 600 cmÿ1 (1978), 15 600±17 600 cmÿ1 (1977), 17 500±20 000 cmÿ1 (1977). [6] F. Martin, R. Bacis, S. Churassy, J. Verges, J. Mol. Spectrosc. 116 (1986) 71±100. [7] D. Cerny, R. Bacis, J. Verges, J. Mol. Spectrosc. 116 (1986) 458±498. [8] T.J. Quinn, Metrologia 30 (1993/94) 523±541. [9] B. Bodermann, Ph.D. Thesis, Hannover, 1999. [10] W. Ubachs, I. Aben, J.B. Milan, G.J. Somsen, A.G. Stuiver, W. Hogervorst, Chem. Phys. 174 (1993) 285±295. [11] A. Arie, S. Schiller, E.K. Gustafson, R.L. Byer, Opt. Lett. 17 (1992) 1204±1206. [12] R.L. Byer, R.L. Herbst, H. Kildal, M.D. Levenson, Appl. Phys. Lett. 20 (1972) 463±466. [13] B. Wellegehausen, K.H. Stephan, D. Friede, H. Welling, Opt. Commun. 23 (1977) 157±161.
M. Klug et al. / Optics Communications 184 (2000) 215±223 [14] D.R. Jones, S.M. O'Shaughnessy, D.C. Laine, Opt. Commun. 131 (1996) 203±210. [15] W. Luhs, M. Hube, U. Schottelius, B. Wellegehausen, Opt. Commun. 48 (1983) 265±269. [16] U. Hefter, J. Eichert, K. Bergmann, Opt. Commun. 52 (1985) 330±335. [17] J.W. Czarske, R. Philipps, I. Freitag, Appl. Phys. B 61 (1995) 243±248. [18] D.C. Gerstenberger, R.W. Wallace, J. Opt. Soc. Am. B 10 (1993) 1681±1683. [19] S. Gerstenkorn, P. Luc, J. Phys. 46 (1985) 355±364. [20] G. Audi, A.H. Wapstra, Nucl. Phys. A 565 (1993) 1±65.
223
[21] R.P. Hackel, S. Ezekiel, PRL 42 (1979) 1736±1739. [22] J.A. Paisner, R. Wallenstein, J. Chem. Phys. 61 (1974) 4317±4320. [23] R. Clark, A.J. McCaery, Mol. Phys. 35 (1978) 609±617. [24] M. Poelker, P. Kumar, Opt. Lett. 17 (1992) 399±401. [25] J.P. Wallerand, F. Burck, B. Mercier, A.N. Goncharov, M. Himbert, C.H. Borde, Eur. Phys. J. D 6 (1999) 63±76. [26] A. Nagel, L. Graf, A. Naumov, E. Mariotti, V. Biancalana, D. Meschede, R. Wynands, Eur. Phys. Lett. 44 (1998) 31±36. [27] U. Hinze, L. Meyer, B.N. Chichkov, E. Tiemann, B. Wellegehausen, Opt. Commun. 166 (1999) 127±132.
Optics Communications 184 (2000) 215±223
www.elsevier.com/locate/optcom
Frequency stable I2 Raman laser excited by a cw frequency doubled monolithic Nd:YAG laser M. Klug a, K. Schulze a, U. Hinze a, A. Apolonskii b, E. Tiemann a, B. Wellegehausen a,* a
b
Institut f ur Quantenoptik, Universit at Hannover, Welfengarten 1, D-30167 Hannover, Germany Institute of Automation and Electrometry, University of Novosibirsk, Novosibirsk 630090, Russian Federation Received 26 April 2000; received in revised form 27 July 2000; accepted 1 August 2000
Abstract Continuous Raman laser operation with 127 I2 , 127 I129 I and 129 I2 molecules, excited by a frequency doubled diode pumped monolithic Nd:YAG ring laser is reported. Within the tuning range of the pump laser, the molecules possess several absorption transitions, which so far enabled laser operation on more than 50 lines in the spectral range of 544± 1330 nm. Multi-line and single-line operation in standing wave and ring resonators have been investigated. With pump powers of up to 500 mW, output powers up to 10 mW on individual lines were possible. Due to the narrow Raman gain pro®les (down to 100 kHz) and the narrow pump laser line width of 10 kHz of the Nd:YAG, correspondingly narrow Raman laser line widths are expected. Therefore, with the pump laser stabilized on a speci®c hyper®ne iodine absorption transition and a frequency locking of the Raman laser resonator to the induced Raman gain pro®le, a system with many reference laser lines and with expected frequency stabilities of dm=m 10ÿ10 can be realized. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 42.65.k; 42.55.Y; 33.55.B Keywords: Nonlinear optics; Frequency stable lasers; Raman lasers; Dynamic Stark eect
1. Introduction Raman type three level laser systems have already been investigated both experimentally and theoretically [1], and cw operation with various diatomic molecules has been demonstrated [2]. Refs. [1,2] give corresponding reviews. Due to the simplicity of the laser scheme, the good conversion eciencies achievable and the easy single fre*
Corresponding author. Fax: +49-511-762-2211. E-mail address: [email protected] (B. Wellegehausen).
quency operation on many lines covering broad spectral ranges, the systems have potential for applications in spectroscopy and metrology. Especially, molecular iodine is an interesting Raman material, because it has a narrow Raman line width [3], and can be pumped with available powerful pump lasers in the green spectral range. It has a dense emission spectrum throughout the visible and near infrared spectral range [4], and can easily be generated at room temperature in sealed-o quartz or glass cells. Furthermore, molecular iodine which is spectroscopically well characterized [4±7] and used for absolute frequency
0030-4018/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 ( 0 0 ) 0 0 9 2 1 - 4
216
M. Klug et al. / Optics Communications 184 (2000) 215±223
stabilization of lasers, and several transitions are recommended as wavelength standard [8,9]. Spectroscopic investigations in the tuning range of the frequency doubled Nd:YAG laser have been done with pulsed [10] and cw [11] lasers. Raman lasers in molecular iodine have been operated with dierent pump lasers, pulsed dye and pulsed frequency doubled Nd:YAG lasers [12], the 514 nm line of the cw argon-ion laser [13,14] and even the low power He±Ne laser line at 633 nm [15]. Instead of using molecular iodine in a cell, cw operation with extremely low threshold was also achieved with a molecular beam as Raman material [16]. In order to use the interesting spectral features of iodine Raman lasers for applications, it seems necessary to replace the dye or argon laser pump source by a more compact and reliable pump source, which in our opinion can be a cw diode pumped frequency doubled Nd:YAG laser. Such lasers are now available with high output powers, and when starting from a monolithic Nd:YAG ring laser, narrow-bandwidth single frequency operation with line widths below 10 kHz is guaranteed [17]. Frequency doubled diode laser pumped monolithic Nd:YAG laser systems have successfully been used for pumping of cw optical parametric oscillators and the generation of narrowband tunable radiation [18]. In this paper, we report cw Raman laser operation in 127 I2 , 129 I2 , 127 I129 I molecular iodine pumped by such an all solid state laser system. Investigations on the absorption transitions, on the laser operation in standing wave and ring resonators and on the line width and frequency stability are presented, and potential applications of the Raman laser system will be discussed.
the temperature of a sidearm. The temperature could be stabilized and varied between 0°C and 40°C. The temperature of the tube was kept slightly higher than that of the sidearm temperature in order to prevent condensation of iodine vapor on the Brewster windows. The experimental setup is shown in Fig. 1. Stokes±Raman lasers have been investigated using (a) simple two mirror standing wave resonators and (b) folded four mirror ring resonators. In the case of the standing wave resonator, the pump radiation is focussed into the resonator through mirror M1, which has a transmission of 90% for the pump radiation and a maximum re¯ection for the Raman laser lines. Frequency selection is realized by inserting a birefringent ®lter (BF) or a high transmittive interference ®lter into the resonator. The generated laser radiation is coupled out via mirror M2 or by an additional quartz plate (QP), which is inserted into the resonator at an angle close to the Brewster angle. The ring resonator consists of two spherical mirrors (M1, M2) and two ¯at mirrors (M3, M4), and a BF as frequency selective element. Mirror
2. Experimental For the laser investigations, dierent types of sealed-o Brewster windowed quartz cells were used. Two tubes with a length of 165 or 185 mm and a diameter of 25 mm were ®lled with spectroscopic grade 127 I2 iodine. Two other laser tubes (length: 250 or 500 mm and diameter: 6 mm) contained mainly 129 I2 and a small amount of 127 I2 , which led to the formation of the mixed 127 I129 I molecule. The vapor pressure was controlled by
Fig. 1. Experimental setup for generating Raman lasers in I2 (a) standing wave resonator and (b) folded ring resonator. M1± M4: mirrors, BF: birefringent ®lter, QP: quartz plate for output coupling, L: focussing lens, F: ®lter for blocking pump radiation, D: photodiode, PZT: piezoelectric transducer, HV: highvoltage supply, NF: low frequency generator, and TK: thermo controller.
M. Klug et al. / Optics Communications 184 (2000) 215±223
M3 is mounted on a low-voltage piezoelectric transducer (PZT) for ®ne adjustment of the cavity length and stabilization of the Raman laser frequency via lock-in technique to the maximum of the gain pro®le.The pump radiation passes mirror M4 (transmission for the pump radiation about 90%) and is focussed by M2 into the vapor cell. All mirrors are highly re¯ective for the generated Raman laser radiation. M1, M2 and M3 are also highly re¯ective for the pump radiation. Output coupling of the generated Raman radiation is done by inserting a QP into the resonator. According to the much stronger forward ampli®cation [1,2], the Raman ring laser oscillates only in the direction of the pump radiation. The standing wave resonator was mainly used for the evaluation of possible pump and Raman laser lines, while the ring resonator was applied for all investigations on frequency stable single line operation.
3. Laser lines and laser operation For a proper identi®cation of possible pump transitions within the tuning range of the pump laser, absorption spectra have been taken for both types of cells simultaneously. Using the wellknown 127 I2 absorption spectrum of the B±X system (level scheme in Fig. 2) as reference [5], the absorption spectra of 129 I2 and 127 I129 I can easily be
217
assigned. The pump laser can be tuned by the temperature of the Nd:YAG laser crystal with a tuning coecient of approximately 8 GHz Kÿ1 ; mode hops occur every 2 K. Within the tuning range of the pump laser (18 787.1±18 789.5 cmÿ1 ) several strong transitions from v00 0 in the ground to v0 32±37 in the B state of the three isotopomeres have been found. The identi®cation of the resonances in the 127 I2 is straightforward using the very well-known molecular parameters [19] or the iodine atlas [5] and is given in Table 1. The identi®cation of the pump transitions in the cell with the mixed isotopomeres is based on the Dunham parameters for 127 I2 , which have been recalculated for 129 I2 and 127 I129 I according to the dierent molecular masses [20]. Assignments can be found in Tables 2 and 3. Table 1 Pump transitions in the tuning range of the frequency doubled Nd:YAG laser in 127 I2 (the line number corresponds to the numbering given in Ref. [5])a Line
Transition
FCF 103
(cmÿ1 )
1106 1107 1108 1109 1110 1111 1112 1113
P119(35-0) R86(33-0) R106(34-0) P83(33-0) R56(32-0) P53(32-0) P142(37-0) R121(35-0)
28.5 30.6 29.6 30.6 31.4 31.4 25.9 28.5
18 787.12 18 787.27 18 787.34 18 787.80 18 788.33 18 788.41 18 788.79 18 789.17
a On transitions indicated in bold, laser operation was investigated.
Table 2 Pump transitions in the tuning range of the frequency doubled Nd:YAG laser in 129 I2 a
Fig. 2. Relevant potential energy curves: within the tuning range of the frequency doubled Nd:YAG laser at 532 nm, pump transitions between v00 0 and v0 32±37 have been identi®ed. Raman laser lines in the range v00 2±72 have been operated.
Transition
FCF 103
(cmÿ1 )
R143(37-0) P115(35-0) R102(34-0) R46(32-0) P43(32-0) R80(33-0) P99(34-0) P77(33-0) R45(32-0) P42(32-0)
26.1 28.7 29.8 31.4 31.4 29.7 28.9 29.7 30.3 30.3
18 787.21 18 787.66 18 788.00 18 788.13 18 788.17 18 788.64 18 788.93 18 789.03 18 789.45 18 789.49
a On transitions indicated in bold, laser operation was investigated.
218
M. Klug et al. / Optics Communications 184 (2000) 215±223
Table 3 Pump transitions in the tuning range of the frequency doubled Nd:YAG laser in 129 I127 Ia Transition
FCF 103
(cmÿ1 )
R52(32-0) P49(32-0) R133(36-0) R120(35-0) R104(34-0) R83(33-0) P80(33-0) R51(32-0) P48(32-0) P101(34-0) P117(35-0)
31.4 31.4 27.4 28.6 29.7 30.7 30.7 31.4 31.4 29.7 28.6
18 787.21 18 787.28 18 787.45 18 787.72 18 787.82 18 788.21 18 788.67 18 788.72 18 788.79 18 788.84 18 789.42
a On transitions indicated in bold, laser operation was investigated.
Strong absorption transitions where laser operation has been investigated are indicated in bold, but all other transitions should also be applicable for laser purposes. The threshold for Raman laser operation is dependent on the losses of the resonator, the cell length, and the vapor pressure. Here, typical pump thresholds around 60 mW in ring resonators were achieved. Above the threshold, the output power ®rst increases linearly with the pump power and then saturates. To achieve maximum output power, the vapor pressure and the output coupling have to be optimized. For strong lines, maximum output powers of about 10 mW at pump powers up to 500 mW were observed so far. The output power for 129 I2 laser lines and mixed isotopomeres lines was typically higher than that for 127 I2 , probably due to the longer cells. In the case of the mixed isotopomere, the vapor pressure seems to be too low to achieve optimum laser operation. From output coupling measurements, gain values of 0.002 cmÿ1 were determined for lasing at wavelengths of 599, 604 and 618 nm in 129 I2 at pump intensities of 500 W cmÿ2 (230 mW at a focal diameter of 240 lm of the pump laser). For a resonator without frequency selection elements, oscillation may occur simultaneously on several lines, depending on the mirror characteristics. However, as all lines share the same upper laser level and as the corresponding narrow Raman gain pro®les are not likely to be simulta-
neously resonant with dierent modes of the resonator, this kind of operation is quite unstable and leads to strong intensity ¯uctuations for the lines. Stable operation is only achieved if a single line is selected by means of a highly selective birefringent or interference ®lter and if the resonator length is stabilized to the gain pro®le. This stabilization and complications, which result from the hyper®ne structure, will be discussed below. With the given pump laser at 532 nm and the corresponding absorption transitions of Tables 1±3, so far Raman laser lines have been operated and spectroscopically identi®ed in the spectral range of 544±1330 nm (Tables 4±6). For measuring the laser lines, a 0.5 m monochromator is used with an accuracy of 0.5 nm. The P and R branches can be resolved over the whole detected spectral region. The values presented in Tables 4±6 are calculated values. To our knowledge, laser lines from the mixed 127 I129 I isotopomere have been observed for Table 4 Realized Raman lasers pumped by R56(32-0) in quencies calculated from Ref. [6]) v00
Frequency (cmÿ1 ) R(56)
P(58)
31 33 35 38 40 42 44 47 49 51 53 54 55 56 57 58 60 62 64 66 67 68 69 71 72
12 803.62 12 465.82 12 134.68 11 650.90 11 337.29 11 031.08 10 732.51 10 299.55 10 021.25 9751.58 9490.86 9363.95 9239.41 9117.28 8997.60 8880.42 8653.72 8437.54 8232.23 8038.10 7945.33 7855.47 7768.53 7603.57 7525.57
12 795.96 12 458.23 12 127.16 11 643.49 11 329.96 11 023.83 10 725.35 10 292.52 10 014.31 9744.74 9484.12 9357.27 9232.78 9110.71 8991.08 8873.96 8647.38 8431.33 8226.15 8032.17 7939.47 7849.69 7762.83 7598.03 7520.12
127
I2 (fre-
FCF 103 7.8 8.8 7.9 5.5 8.6 10.2 9.6 6.2 10.3 13.1 13.5 2.1 11.5 6.7 7.7 12.8 19.2 24.5 27.0 24.9 24.2 14.7 64.2 130.0 210.0
M. Klug et al. / Optics Communications 184 (2000) 215±223 Table 5 Realized Raman lasers pumped by P77(33-0) in cies calculated from Ref. [7]) v00 2 4 5 6 7 9 11 13 15 17 22 24 26 28 31 33 35 37 38 39 40 42 44 46 48
Frequency (cmÿ1 ) R(75)
P(77)
18 379.45 17 963.50 17 757.39 17 552.52 17 348.90 16 945.46 16 547.13 16 154.01 15 766.18 15 383.72 14 451.75 14 088.92 13 731.97 13 381.03 12 866.19 12 530.93 12 202.22 11 880.26 11 721.87 11 565.25 11 410.40 11 106.22 10 809.54 10 520.62 10 239.75
18 368.08 17 952.42 17 746.34 17 541.51 17 337.92 16 934.55 16 536.30 16 143.25 15 755.49 15 373.11 14 441.33 14 078.59 13 721.72 13 370.86 12 856.16 12 520.99 12 192.37 11 870.51 11 712.17 11 555.60 11 400.80 11 096.72 10 800.15 10 511.35 10 230.59
129
I2 (frequen-
FCF 103 13.1 7.8 5.5 4.5 7.0 8.2 8.7 8.4 7.3 5.5 7.2 7.9 7.3 5.6 6.1 8.0 8.5 7.5 2.5 5.2 5.4 8.3 10.0 10.0 8.2
the ®rst time. From the potential curve of the iodine molecule (Fig. 2), laser oscillation should be possible up to wavelengths of about 1.5 lm. Infrared laser lines up to 1.35 lm v00 72 have been observed. Longer wavelengths could not be detected so far due to limitations of our monochromator and detector system. For determination of the observed Raman transitions to v00 > 19 in the ground state, the data set from Bacis and coworkers [4,6,7] is used for frequency calculation.
4. Single line operation and line width of free running Raman lasers To achieve single frequency operation of the iodine Raman laser, in addition to the selection of a single rovibronic transition as discussed above, a single hyper®ne component on the pump and Raman transition has to be selected. The pump
219
Table 6 Realized Raman lasers pumped by P80(33-0) in quencies calculated from Ref. [7]) v00 4 6 7 9 11 13 15 17 18 22 24 26 28 33 35 37 39 42 43 44 46
Frequency (cmÿ1 ) R(78)
P(80)
17 960.67 17 548.24 17 343.92 16 939.09 16 539.43 16 145.01 15 755.95 15 372.28 15 182.52 14 437.54 14 073.69 13 715.77 13 363.92 19 135.08 19 135.08 19 135.08 11 544.31 11 084.62 10 935.15 10 787.62 10 498.47
17 949.06 17 536.71 17 332.42 16 927.67 16 528.08 16 133.75 15 744.76 15 361.18 15 171.45 14 426.63 14 062.87 13 705.05 13 353.28 19 135.08 19 135.08 19 135.08 11 534.21 11 074.69 10 925.28 10 777.80 10 488.77
127 129
I
I (fre-
FCF 103 8.8 5.5 6.0 7.4 8.3 8.5 7.8 6.4 2.5 6.5 7.7 7.8 6.6 7.2 8.5 8.3 6.6 7.0 1.5 9.5 10.5
laser can be stabilized on hyper®ne components by means of FM saturation spectroscopy [11]. The selected hyper®ne pump transition then leads to gain on only one speci®c hyper®ne component of the Raman transition, according to the selection rule DF DJ for J P 20. The situation for the transition considered below for the line width investigations is explained in Fig. 3. Within the Doppler pro®le of the selected hyper®ne component (about 270 MHz at T 300 K), other hyper®ne components may also be excited o-resonance, which may lead to o-resonance oscillation on additional hyper®ne components. This situation can be improved by selecting isolated hyper®ne pump transitions (e.g. a1 , a10 , a15 ). Comparable isolated hyper®ne components can be found for all transitions with high even J values, but R56(32-0) is especially favorable for us. For investigations of the line width and frequency stability of the Raman laser, the Raman transition R56(31-32) at 12 803.6 cmÿ1 was chosen. For this Raman transition, the Franck±Condon
220
M. Klug et al. / Optics Communications 184 (2000) 215±223
Fig. 4. Shape of the output power pro®les at dierent pump intensities: (a) 260 W cmÿ2 , (b) 3 kW cmÿ2 , and (c) 4 kW cmÿ2 . The threshold pump intensity is less than 200 W cmÿ2 .
Fig. 3. Example of pumping a single hyper®ne line (e.g. R56(320) a10 [11]) by a frequency doubled Nd:YAG laser solid line of transition j1i±j3i. Adjacent lines (dotted) are D 150 MHz apart and are pumped o-resonant within the indicated Doppler pro®le. On the Raman transition j3i±j2i, the induced narrow forward gain pro®les [2,21] are clearly separated and the central component (solid line) is favored (D0 120 MHz) for R56(31-32).
factor (FCF) has a local maximum; standard photodiodes are most sensitive and a good set of mirrors was available. To investigate the line width and frequency stability of the Raman laser, two almost identical ring laser systems (Fig. 1b) have been built in order to allow beat frequency measurements. As the Raman gain pro®le can be very narrow (less than 80 kHz at vapor pressures of 0.07 Pa and pump intensities of 5 mW cmÿ2 [21]), the length of the resonator has to be controlled and stabilized in order to keep one mode of the resonator within the pro®le (mode spacing 150 MHz). Single frequency operation can thus be controlled by using the resonator itself as a scanning Fabry±Perot interferometer. In addition, by scanning the resonator length and measuring the output power of the Raman laser, an output power frequency pro®le is obtained from which a rough estimation of the width of the Raman gain pro®le may be obtained (see discussion below). Such measurements are shown
in Fig. 4 for dierent pump intensities. At low pump intensity (close to threshold), a narrow single peak pro®le with a width of about 4.1 MHz (FWHM) is obtained, while at higher pump intensities, the pro®les are split due to the dynamic Stark eect. For the line width and frequency measurement, the intensity was kept low, in order to avoid the splitted pro®le. For locking one mode of the Raman laser resonator to the induced hyper®ne power pro®le, the position of M3 is modulated by a PZT with a frequency of 1 kHz (Fig. 1b). The corresponding frequency modulation depth has to be smaller than the width of the gain pro®le. The resulting amplitude modulation of the output power is then detected by photodiode D and demodulated by a lock-in, which corrects the oset of the PZT to ®x the laser to the maximum of the gain pro®le. In this way, amplitude stabilities of the laser system of better than 10% over minutes are reached. Without the locking, the laser switches on and o within time intervals of a few seconds, indicating strong frequency drifts and frequency ¯uctuations. To measure the laser line width, parts of the laser emission (100 lW) of both lasers are overlapped on a beamsplitter and focussed onto an avalanche photodiode. The detected RF spectrum is analyzed by a spectrum analyzer (HP4395). An example of the RF spectrum is shown in Fig. 5. The spectrum shows a width of the beat signal of 4.5 kHz (sweep time: 220 ms) from which a contribution of each Raman laser of 3.2 kHz may be deduced assuming statistically independent la-
M. Klug et al. / Optics Communications 184 (2000) 215±223
Fig. 5. Measured beat spectrum of the two Raman lasers: the resolution bandwidth is 1 kHz and the sweep time is 220 ms.
sers. This line width is smaller than the free running line width of about 10 kHz in 100 ms of the pump laser. For the interpretation of this result, a brief discussion of factors in¯uencing the Raman gain pro®le and with it the power pro®le and the Raman laser frequency stability are helpful. The small signal Raman gain pro®le at resonance conditions in general has a complicated structure and a forward±backward asymmetry due to the superposition of single- and two-photon contributions. For a more detailed discussion of the gain pro®les see Refs. [1,2]. For low pump intensities (no power broadening and no dynamic Stark splitting) and considering the forward direction, a nearly Lorentzian type pro®le may be obtained, which is assumed here for simplicity, with a center frequency x0R x32 kP =kR x0P ÿ x31 and a spectral width (FWHM) DXR of kP kP Dx23 ; DXR Dx12 1 ÿ kR kR where x31 and x32 are the transition frequencies on the 3±1 pump transition and on the 3±2 Raman transition. x0P is the pump frequency; Dx23 and Dx12 are the homogenous pressure broadened line width of the single photon transition 3±2 and the two-photon transition 2±1, respectively, and 1 ÿ kP =kR is the Doppler compensation factor with kP and kR being the pump and Raman laser wavelengths. In the case of iodine, the corre-
221
sponding values can be calculated using the relaxation rate and collision cross-sections given in Refs. [22,23], and yield for the above considered transition, Dx12 2:0 MHz and Dx32 4:1 MHz (at p 27 Pa) and DXR 2:6 MHz. Experimentally (Fig. 4) power pro®les were measured, which only indirectly re¯ect the gain pro®les, as under lasing conditions saturation and propagation effects have to be considered. The width of 4.1 MHz of the power pro®le at low pump intensity (Fig. 4a) best approaches the above situation and is in good agreement with the calculated value of 2.6 MHz. A ¯uctuation of the pump frequency around x0P x0P x31 will move the pro®le and broaden it when averaged in time. If the ¯uctuating frequency of the mode of the resonator is ideally (i.e. instantaneously or the bandwidth of the stabilization circuit is suciently wide) locked onto the center frequency, x0R , ¯uctuations of the Raman resonator will be fully removed, and the Raman laser frequency will follow the pump frequency with the reduction factor kP =kR and consequently will have a corresponding line width. From the 10 kHz width of the pump laser, we can therefore expect at best a spectral width of one Raman laser of 532 nm=781 nm 10 kHz 7 kHz under these conditions. For a non-instantaneous locking, only part of the mode ¯uctuations can be removed and only part of the pump frequency ¯uctuations can be followed. Consequently, the resulting Raman laser line width will then contain contributions from the pump laser line width and from the Raman laser resonator itself in addition to the contribution from the frequency modulation. In the described beat frequency experiment, both Raman lasers are pumped by the same pump laser, which means that they are no more statistically independent. It is expected that the Raman lasers will have correlated frequency ¯uctuations resulting from the pump laser and stabilization circuit, which to a ®rst approximation will disappear in the beat spectrum. This is also indicated by the fact that for these experiments, a free running (drifting) pump laser could be used, without any in¯uence on the beat frequency spectrum. The measured line width of the beat signal therefore mainly represents the line width contribution due to the short-term ¯uctuations from the Raman
222
M. Klug et al. / Optics Communications 184 (2000) 215±223
laser setup itself. These may be further reduced by a more stable setup and with a larger bandwidth of the stabilization circuit. In order to measure the real Raman laser line width (including contributions of the pump laser) either beat measurements with independently pumped Raman lasers or direct measurements with a high ®nesse cavity have to be performed. Such experiments are in progress. The present investigations, however, already show that narrow line width Raman lasers are possible. Therefore, with a stable pump laser locked to an iodine hyper®ne transition, correspondingly stable Raman lasers may be realized for all transitions having FCF above 5 10ÿ3 (Tables 4±6). 5. Conclusions In this paper, investigations on iodine Raman lasers pumped by a stable narrowband frequency doubled Nd:YAG laser are presented. Within the tuning range of the pump laser, at least 29 pump transitions for 127 I2 , 127 I129 I and 129 I2 isotopomeres exist, which allow Raman laser oscillation. Raman laser lines could be realized in the spectral range of 544±1330 nm, for the ®rst time also in the 127 I129 I isotopomere. All pump and laser transitions could be assigned, and based on molecular data, wave numbers for all involved transitions were calculated with a high accuracy. So far, performed power pro®le and line width measurements demonstrate that narrow line width (kHz) Raman lasers in iodine can be operated. With the present pump laser system, stabilized to an iodine hyper®ne transition, it is possible to operate Raman lasers with a relative frequency stability of dm=m 10ÿ10 for all transitions having FCF P 5 10ÿ3 . Concerning the absolute stability, further long-term stability investigations are necessary, and especially the role of the dynamic Stark splitting has to be studied. Pump and Raman lasers perform a two-photon transition, which leads to a coupling of the ®eld frequencies. The dierence frequency of these coupled (bichromatic) ®elds may have a much smaller line width as the incident ®elds [24], which is of interest in metrology and high precision spectroscopy.
Bichromatic ®elds with small dierence frequencies (GHz range) have successfully been applied in high precision hyper®ne measurements [25] and in preparing dark resonances [26]. Raman coupled bichromatic ®elds generated in the Na2 molecule have recently been applied by us for ef®cient dierence frequency mixing [27] and demonstration of parametric ampli®cation. By the Raman process in iodine stable dierence frequency ®elds in a wide range of dierence frequencies may be generated. Investigations on line width and frequency stability applying a direct representation of the bichromatic ®eld in a nonlinear crystal are in progress.
Acknowledgements This work was supported by the Deutsche Forschungsgemeinschaft within the SFB 407.
References [1] I.M. Beterov, V.P. Chebotaev, Progress in Quantum Electronics, vol. 3, Part 1, Pergamon Press, New York, 1974, p. 1. [2] B. Wellegehausen, IEEE J. Quant. Electron. QE15 (1979) 1108±1130. [3] J.B. Koend, S. Goldstein, R. Bacis, R.W. Field, S. Ezekiel, PRL 41 (1978) 1040±1043. [4] R. Bacis, D. Cerny, F. Martin, J. Mol. Spectrosc. 118 (1986) 434±447. [5] S. Gerstenkorn, P. Luc, Atlas du Spectre d'Absorption de la Molecule d'Iode, Laboratoire Aime Cotton, CNRS II, 91405 Orsay, France, 14 000±15 600 cmÿ1 (1978), 15 600±17 600 cmÿ1 (1977), 17 500±20 000 cmÿ1 (1977). [6] F. Martin, R. Bacis, S. Churassy, J. Verges, J. Mol. Spectrosc. 116 (1986) 71±100. [7] D. Cerny, R. Bacis, J. Verges, J. Mol. Spectrosc. 116 (1986) 458±498. [8] T.J. Quinn, Metrologia 30 (1993/94) 523±541. [9] B. Bodermann, Ph.D. Thesis, Hannover, 1999. [10] W. Ubachs, I. Aben, J.B. Milan, G.J. Somsen, A.G. Stuiver, W. Hogervorst, Chem. Phys. 174 (1993) 285±295. [11] A. Arie, S. Schiller, E.K. Gustafson, R.L. Byer, Opt. Lett. 17 (1992) 1204±1206. [12] R.L. Byer, R.L. Herbst, H. Kildal, M.D. Levenson, Appl. Phys. Lett. 20 (1972) 463±466. [13] B. Wellegehausen, K.H. Stephan, D. Friede, H. Welling, Opt. Commun. 23 (1977) 157±161.
M. Klug et al. / Optics Communications 184 (2000) 215±223 [14] D.R. Jones, S.M. O'Shaughnessy, D.C. Laine, Opt. Commun. 131 (1996) 203±210. [15] W. Luhs, M. Hube, U. Schottelius, B. Wellegehausen, Opt. Commun. 48 (1983) 265±269. [16] U. Hefter, J. Eichert, K. Bergmann, Opt. Commun. 52 (1985) 330±335. [17] J.W. Czarske, R. Philipps, I. Freitag, Appl. Phys. B 61 (1995) 243±248. [18] D.C. Gerstenberger, R.W. Wallace, J. Opt. Soc. Am. B 10 (1993) 1681±1683. [19] S. Gerstenkorn, P. Luc, J. Phys. 46 (1985) 355±364. [20] G. Audi, A.H. Wapstra, Nucl. Phys. A 565 (1993) 1±65.
223
[21] R.P. Hackel, S. Ezekiel, PRL 42 (1979) 1736±1739. [22] J.A. Paisner, R. Wallenstein, J. Chem. Phys. 61 (1974) 4317±4320. [23] R. Clark, A.J. McCaery, Mol. Phys. 35 (1978) 609±617. [24] M. Poelker, P. Kumar, Opt. Lett. 17 (1992) 399±401. [25] J.P. Wallerand, F. Burck, B. Mercier, A.N. Goncharov, M. Himbert, C.H. Borde, Eur. Phys. J. D 6 (1999) 63±76. [26] A. Nagel, L. Graf, A. Naumov, E. Mariotti, V. Biancalana, D. Meschede, R. Wynands, Eur. Phys. Lett. 44 (1998) 31±36. [27] U. Hinze, L. Meyer, B.N. Chichkov, E. Tiemann, B. Wellegehausen, Opt. Commun. 166 (1999) 127±132.