slave injection-locking of laser diodes

Available online at www.sciencedirect.com

Optics & Laser Technology 37 (2004) 81 – 86 www.elsevier.com/locate/optlastec

Spectral characteristics of cascade master/slave/slave injection-locking of laser diodes I. Hirano∗ , N. Ito National Metrology Institute of Japan, AIST, Tsukuba Central 3 Umezono 1-1-1 Tsukuba, Ibaraki, Japan Received 1 September 2003; received in revised form 22 March 2004; accepted 24 March 2004

Abstract We have developed the master/slave/slave laser spectrometer in the blue-wavelength region. It comprises a master/slave laser combination coupled with a second-harmonic generation (SHG) enhancement cavity and a SHG-injected blue diode laser. The total power of 30 mW was obtained and the spectral characteristics and power were measured. Excluding the power of the satellite longitudinal mode of a slave/slave laser, the total power of 422:791 nm light which can be used for the magnetooptic trap for Ca was 25:5 mW. ? 2004 Elsevier Ltd. All rights reserved. Keywords: Injection-locking; Laser diode; Spectrum

1. Introduction There has been much interest recently in the use of diode lasers in laser cooling and trapping as well as high-resolution atomic and molecular spectroscopy. Diode lasers are attractive sources of coherent light for atomic physics because of their small size, low cost, continuous wavelength tenability, and reasonable output powers with very high electrical to optical e
Corresponding author. E-mail address: [email protected] (I. Hirano).

0030-3992/$ - see front matter ? 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2004.03.014

reduced if they are used in an extended-cavity conBguration to improve the spectral purity [10]. Conversely, there are high-power devices in which the width of the active layer has been increased at the cost of a well-deBned transverse mode pattern. In the infrared spectral range, injection locking techniques [11] have been used to combine the spectral and spatial properties of low-power single-mode lasers with high-power diode lasers or even diode arrays [12]. One of the greatest technical challenges in building a diode-based magnetooptic trap (MOT) for Ca is the generation of single-frequency, tunable radiation at 423 nm with su
2. Experimental set-up A schematic diagram of our experimental set-up is shown in Fig. 1. The master/slave laser combination coupled with a crystal for SHG is similar to that used for a previous NRLM work using Rb which is described in detail elsewhere [14]. The master laser is an external cavity diode laser (ECDL) that comprises an antireCection-coated laser diode (SDL-AR-5412-H1), a collimating lens, a

82

I. Hirano, N. Ito / Optics & Laser Technology 37 (2004) 81 – 86

Fig. 1. Experimental set-up: PZT, piezoelectric transducer; KNbO3 , potassium niobate; PBS, polarizing beam splitter; PD, photodiode; =2, half-wave plate; =4, quarter-wave plate; P.P., anamorphic prism pair; H.V. amp, high-voltage ampliBer.

diKraction grating at grazing incidence, and an external mirror in the Littman–Metcalf conBguration [15]. The power injected from the master to the slave laser is 5 mW. The slave laser is a single-stripe high-power laser diode (SDL-5432-H1; ¡ 200 mW) without antireCection coating, followed by a collimating lens. The injection-locked slave laser provides a power of 150 mW. Cylindrical lenses with focal lengths of 80 and 15 mm are set before and after the isolator, respectively. They are set in order to modify the excessively oblong cross section of the collimated beam output from the slave laser as well as to reduce the beam cross section so that it will be transmitted entirely through the isolator. In front of the cavity, a mode-match lens of 300 mm focal length is used after reshaping the cross section of the slave laser beam to a quasi-circular shape by means of an anamorphic prism pair. The beam from the slave laser is introduced to a KNbO3 crystal for an SHG setup within a triangular enhancement cavity. The total length of the enhancement cavity is 755 mm, and its free spectral range (FSR) is approximately 400 MHz. A pair of lenses (f = 40 mm) provides a small beam waist. The crystal is enclosed in a small vacuum box to prevent frost formation on its surface when its temperature is lowered to −8◦ C to accomplish noncritical type-I phase matching for the 41 S0 –41 P1 transition line. The surfaces of box windows, lenses, and the crystal are all antireCection (AR)-coated against both fundamental and second-harmonic radiations. If the mode matching is thorough, the power-enhancement factor is determined by (1 − Rc )=[1 − Rc0:5 (1 − )0:5 ]2 , where Rc and  are the reCectivity of the input coupler mirror and an additional intracavity optical loss, respectively [16]. The two other mirrors have the reCectivity of 0.991 and 0.976. The transmission of the crystal is 0.991. From these values we assumed that the intrinsic loss of the cavity/crystal is

Fig. 2. Calculated enhancement factors plotted against the reCectivity of an input coupler for each value of internal loss (). The dotted line shows the most e
0.042. Fig. 2 shows the enhancement factor plotted against the reCectivity of an input coupler. The dotted line shows the most e
I. Hirano, N. Ito / Optics & Laser Technology 37 (2004) 81 – 86

83

Table 1 Maximum enhancement factors obtained with the optimal reCectivity of the input coupler (R) against an assumed intrinsic loss of the cavity ()

 R Enhancement (max.)

0.05 0.95 20

0.04 0.96 25

0.03 0.97 33.3

0.02 0.98 50

coupler with reCectivity of 0.95 (a slightly smaller value than the optimum reCectivity of 0.96) the calculated enhancement is 24.7, which is a slightly smaller value than the maximum enhancement of 25. On the basis of these results we select the value of 0.95 for the input mirror. As Rc is about 0.95, this enhancement is expected to be 25, if we assume  of 0.04. Finesse of the cavity (F; ratio of one FSR to the transmission bandwidth) is inferred to be 35 based on the observed transmission proBle of the cavity. From this, the round-trip reCectivity R is estimated to be 0.91 simply based upon the relation F = ( R0:5 )=(1 − R), and consequently,  of approximately 0.04 is deduced [17]. This value is coincides with the product of the reCectivities of the three mirrors and the transmission of the crystal. Attention was paid to the selection of an appropriate beam waist size and crystal length. In most cases the SHG e
Fig. 3. Frequency discrimination signals detected by means of HMansch– Couillaud method. ◦ and  are with the main modes and the associated transverse modes, respectively.

of the associated transverse modes, and su
84

I. Hirano, N. Ito / Optics & Laser Technology 37 (2004) 81 – 86

Fig. 4. Spectra of the confocal Fabry–Perot interferometer, (a) slave laser and (b) SHG.

Fig. 5. Emission spectra of the NHLV3000E with and without the injection SHG beam. (a) I = 60:9 mA, Pin = 1:2 mW, Pout = 7:7 mW. (b) I = 72:0 mA, Pin = 1:2 mW, Pout = 17:7 mW.

spectral range is 2 GHz. The spectra of the SHG was broadened due to the low Bnesse of the Fabry–Perot interferometer. When an injection SHG beam was used, the slave/slave blue laser oscillated in the quasi-single mode, as shown in Fig. 5. Figs. 6 and 7 show the spectra of the slave/slave

blue laser which were observed by using the confocal Fabry –Perot interferometer. Fig. 6 corresponds to Fig. 5(a) and Fig. 7 corresponds to Fig. 5(b). In both Figs. 6 and 7, the left-hand side photo shows the spectrum for the case that the blue laser was not injection locked and the right-hand side photo shows the spectrum for the case that the blue laser was injection locked. The contrast of the transmission spectrum of the confocal Fabry–Perot interferometer became clear when the blue laser was injection locked. The value of the wavemeter changed from 421.722 to 422:791 nm at 72:0 mA. As shown in Fig. 4(b), the output power increased from 10.6 to 17:7 mW, that is, by approximately 70%, when an injectioned SHG beam was used. The transmittance of the isolator was 0.62. In this system comprising the blue laser, =2 wave plate and isolator, 11:0 mW output power was obtained with 2:2 mW input slave SHG power. Excluding the power of the satellite longitudinal mode of the slave/slave laser, the total power of 422:791 nm light which can be used for the magnetooptic trap (MOT) for Ca was 25:5 mW. To conBrm the injection locking to the slave/slave blue laser, we sent the injection-locked blue laser through an acoustooptic modulator and observed the beat signal between the SHG and the 40 MHz frequency-shifted injection-locked blue laser. The observed beat signal is shown in Fig. 8. Since NHLV3000E has a multi-quantum-well (MQW)/GaN/AlGaN separate conBnement heterostructure, it oscillates in the multilongitudinal

Fig. 6. Spectra of the slave/slave blue laser. Injection current of the blue laser was 60:9 mA, (a) free run and (b) injection locked (input power was 1:2 mW and output power was 7:7 mW).

I. Hirano, N. Ito / Optics & Laser Technology 37 (2004) 81 – 86

85

Fig. 7. Spectra of the slave/slave blue laser. Injection current of the laser was 72:0 mA, (a) free run and (b) injection locked (input power was 1:2 mW and output power was 17:7 mW).

Table 2 The intensity ratio of each satellite mode against the main longitudinal mode

Injection current (mA) 60.9 72.0 72.0

SHG injection locked ◦ × ◦

−0:55 (nm)

−0:44 (nm)

−0:33 (nm)

−0:22 (nm)

0.22 0.24 0.23

0.19 0.21 0.19

0.14 0.14 0.13

0.08 0.09 0.08

The values −0:55; −0:44; −0:33, and −0:22 indicate how much shorter the wavelength was from the main longitudinal wavelength.

Fig. 8. Observed beat signal between the SHG and 40 MHz frequencyshifted-injection locked blue laser.

mode [18] 1 as shown in the lower spectrum of Fig. 5(a). The resolution of the monochromator was 0:1 nm, which could distinguish each longitudinal mode of NHLV3000E. However, if we adequately adjust the temperature and current, the laser oscillates in a quasi-single mode, as shown in the lower spectrum of Fig. 5(b), in which 4 satellite longitudinal modes were observed on the short-wavelength side of the main spectrum. This phenomenon was caused by interference eKects between the light reCected from the glass of the package of NHLV3000E and that from the Beld inside the laser diode, since the wavelength of the laser changes depending on the injection current and temperature [19]. As shown in Fig. 5(b), when the laser was injection locked, the spectra shifted to a longer wavelength by about 1 nm. Both the main longitudinal and satellite longitudinal modes shifted to a longer wavelength by about 1 nm. The possible injection locking range was 72:0 ± 0:5 mA at 23:7◦ C. When the injection current exceeded this range, NHLV3000E was not injection locked to SHG. Also as shown in Fig. 5(a), NHLV3000E was injection locked to SHG when the injection current was 60:9 ± 0:5 mA at 23:7◦ C. When the injection current exceeded this range, NHLV3000E was not injection locked to SHG. Table 2 shows the intensity ratio 1

Private talk with an employee of Nichia Corporation, 2004.

of each satellite mode against the main longitudinal mode. The values −0:55; −0:44; −0:33, and −0:22 indicate how much shorter the wavelength was form the main longitudinal wavelength. The intensity ratio of satellite mode against the main longitudinal mode was almost constant regardless of current. 4. Conclusion The master/slave/slave laser spectrometer in the bluewavelength region has been developed, comprising a master/slave laser combination coupled with a SHG enhancement cavity and an SHG-injected blue diode laser. The spectral characteristics and the power were measured. Excluding the power of the satellite longitudinal mode of the slave/slave laser, the total power of 422:791 nm light which can be used for the magnetooptic trap (MOT) for Ca was 25:5 mW. References [1] Gertsvolf M, Rosenbluh M. Injection locking of a diode laser locked to a Zeeman frequency stabilized laser oscillator. Opt Commun 2000;184:457–62. [2] Repasky KS, Roos PA, Meng LS, Carlsten JL. AmpliBed output of a frequency chirped diode source via injection locking. Opt Eng 2001;40:2505–9.

86

I. Hirano, N. Ito / Optics & Laser Technology 37 (2004) 81 – 86

[3] Kobayashi S, Kimura T. Injection locking in AlGaAs semiconductor laser. IEEE J Quantum Electron 1981;QE 17:681–9. [4] Lang R. Injection locking properties of a semiconductor laser. IEEE J Quantum Electron 1982;QE 18:976–83. [5] Kurosu T, Ishikawa J, Ito N. Diode laser spectrometer for highresolution spectroscopy in the visible range. Appl Phys B 1996;63:265–75. [6] Spano P, Piazzolla S, Tamburrini M. Frequency and intensity noise in injection-locked semiconductor lasers. IEEE J Quantum Electron 1986;QE 22:427–35. [7] Lidoyne O, Gallion PB, Eransme D. Modulation properties of an injection-locked semiconductor laser. IEEE J Quantum Electron 1991;QE 27:344–51. [8] Li L. Static and dynamic properties of injection-locked semiconductor lasers. IEEE J Quantum Electron 1994;QE 30:1701–8. [9] Hui R, Benedetto S, Monitrosset I. Optical bistability in diode-laser ampliBers and injection-locked laser diodes. Opt Lett 1993;18: 287–9. [10] Celikov A, Riehle F, Velichansky VL, Helmcke J. Diode laser spectroscopy in a Ca atomic beam. Opt Commun 1994;107: 54–60. [11] Bouyer PH. Spectral stabilization of an InGaAsP semiconductor laser by injection-locking. Ann Phys France 1993;18:89–239.

[12] Tsuchida H. Tunable, narrow-linewidth output from an injectionlocked high-power AlGaAs laser diode array. Opt Lett 1994;19: 1741–3. [13] Oates CW, Bondu F, Fox RW, Hollberg L. A diode-laser optical frequency standard based on laser-cooled Ca atoms: sub-kilohertz spectroscopy by optical shelving detection. Eur Phys J D 1999;7: 449–60. [14] Ito N. Doppler-free modulation transfer spectroscopy of rubidium 52 S1=2 − 62 P1=2 transitions using a frequency-doubled diode laser blue-light source. Rev Sci Instrum 2000;71:2655–62. [15] Harvey KC, Myatt CJ. External-cavity diode laser using a grazing-incidence diKraction. Opt Lett 1991;16:910–2. [16] Jundt DH, Feijer MM, Byer RL, Norwood RG, Bordui PF. 69% e