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Optical feedback locking of a diode laser using a cesium Faraday filter
Optics Communications 96 (1993) 240-244 North-Holland
OPTICS C O M MUN ICAT1ONS
Optical feedback locking of a diode laser using a cesium Faraday filter K y u n a m C h o i , J. M e n d e r s , P a u l S e a r c y a n d E r i c K o r e v a a r Thermo Trex Corporation, 9550 Distribution Avenue. San Diego, CA 92121-2305, USA
Received 9 July 1992; revised manuscript received 6 October 1992
We used weak filtered optical feedback to set the output frequency of a commercial single mode diode laser close to the peak of an atomic line filter passband. A Cs Faraday atomic line filter provided ultra-narrow optical feedback passbands near 852 nm. The locked laser power of 34 mW was nearly that of the unlocked laser. In-band laser operation was tolerant of laser diode current and temperature fluctuations and feedback path instabilities, important qualities in the design of matched filtered optical transceivers. We describe the feedback locked laser diode and present locked spectrum measurements. We show how fine tuning over a 100 MHz range is available by feedback mirror positioning.
1. Introduction
Extremely n a r r o w b a n d single m o d e d i o d e lasers have been developed for applications that require matched transceivers, such as remote sensing a n d optical c o m m u n i c a t i o n s . Optical feedback locking, where filtered feedback controls the o u t p u t frequency, is often used to obtain single m o d e operation. F a b r y - P 6 r o t cavities, gratings, birefringent filters and other mechanically tunable n a r r o w b a n d filters are c o m m o n l y used as the feedback filters [ 1 ]. To take advantage o f state-of-the-art n a r r o w b a n d filters (such as atomic line filters) with 1 G H z passbands in the optical receiver, the absolute frequency o f the laser must be set with a precision o f about a h u n d r e d MHz. The mechanically tunable filters m e n t i o n e d above can achieve this degree o f precision only with the a d d e d complexity of a servo-loop referred to a spectroscopic standard. F o r applications free to operate at the p a s s b a n d frequencies of an atomic line filter, all-optical lockers with intrinsic frequency references, such as those based on resonant phase conjugation [2] and resonant magnetooptic effects (refs. [3,4] and this w o r k ) , are advantageous. The F a r a d a y filter, an o p t i m i z e d resonant magneto-optic filter described in more detail below, has been used as an intraeavity element to frequency lock dye lasers to the s o d i u m D lines [5,6]. Re240
cently, a diode laser in an external cavity was frequency locked by an intra-cavity F a r a d a y filter [ 2 ]. In other recent work, a diode laser was locked using a weekly transmitting magneto-optic filter to provide filtered feedback [4], reducing the usable output power. By using the F a r a d a y filter to provide filtered feedback, we show that frequency control can be achieved using the filter outside the cavity without a significant reduction in the usable laser power. In this Letter, we describe our technique for optical feedback locking a single mode commercially available diode laser using a cesium F a r a d a y filter at 852 nm. To show that the locked laser was well m a t c h e d to a F a r a d a y filtered receiver with a gigahertz bandwidth, we measured the laser diode spect r u m with a resolution of about 40 M H z using a calibrated heterodyne diagnostic. The output power of the locked laser diode of 34 m W was comparable to that of the unlocked laser o f 40 mW. We also found that the laser frequency occupied a resonance frequency associated with the cavity formed by the feedback arm and the diode laser. Thus, a fine adj u s t m e n t of the laser frequency can be made by adjusting the feedback arm length.
0030-4018/93/$ 06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
Volume 96, number 4,5,6
OPTICS COMMUNICATIONS
2. Feedback locking technique As shown in fig. 1, feedback locking was accomplished by sampling the laser output, filtering the sampled beam using a Faraday filter, and retro-reflecting the beam into the laser. Using a Faraday filter in conjunction with a laser diode was possible because the weak sampled beam intensity did not disrupt the Faraday filter which can be spoiled by intense beams as described below. Generally, broadband feedback obtained without the filter caused multimode operation of the unlocked diode laser. Under this condition, the laser emission occurred over a band of some twenty modes separated by a mode spacing of about 100 GHz. The band could be temperature a n d / o r current tuned to straddle the Faraday filter passband near 852 nm. The susceptibility of the diode to external feedback proved to be so great, that very weak feedback levels served to control the laser frequency. A feedback level of about ½% was obtained by double passing a 15% beamsplitter and a 50% Faraday filter transmission peak. We had enough excess feedback power to use the Faraday filter without concern for optimizing its transmission, and to make use of a 2 m convex feedback mirror to ease the alignment tolerance. The feedback easily dominated the weak resonance con-
I
ii
o
] ~~~ 0 [?2
iiPY ; ~1~ i FM I:! FEEDBACK LOCKED LASER
HETERODYNE DETECTION
Fig. 1. Schematic of the optical feedback locked laser cavity configuration and the heterodyne detection setup. The symbols are DL: diode laser; Px, Py: x, y polarizers; Ff: Faraday filters; FM: feedback mirror, O1: optical isolator and D 1, D2: photodiodes.
15 February 1993
dition imposed by the low reflectivity of the uncoated diode facets and the laser spectrum jumped to a position inside the 1 GHz Faraday filter passband. Under these feedback conditions, a narrow laser linewidth of ~<30 MHz was measured using the heterodyne diagnostic. With the 15% feedback arm sampling, we obtained an output power of 34 mW independent of whether the feedback was used to lock the laser or blocked. Further optimization of the Faraday filter transmission can be traded for reductions in the feedback arm sampling, increasing the locked laser output. Within the Faraday filter passband, the laser oscillated in a mode of the cavity formed by the diode and the retro-reflector. To assure that the laser oscillated in a single one of these modes, we configured the feedback cavity length to be about 65 cm to obtain a mode spacing of about 230 MHz, which allowed the Faraday filter passband to discriminate between adjacent modes. For these experiments we used a Spectra Diode Labs SDL-5400-GI CW single mode GaA1As diode laser with a specified bandwidth of ~ 15 MHz. This particular laser emitted about 50 mW at 859 nm for a 71 mA current at room temperature. In order to shift the output to the Cs Faraday filter passband near 852 nm, we operated the laser at about - 1.5°C and 56 mA to produce 40 mW (without feedback locking). Since these lasers are supplied with uncoated facets (which provide an outcoupling of about 75%), they are generally very susceptible to feedback. The Faraday filter, a magneto-optic filter, essentially consists of a resonant Faraday rotator sandwiched between two crossed polarizers. The Faraday rotator usually consists of an alkali metal vapor immersed in an axial magnetic field to provide rotary power in the vicinity of strong absorption lines. The magnetic field Zeeman splits the absorption line and the corresponding dispersion in the refractive index into right and left circular components giving circular dichroism and birefringence. At the fields and temperatures we use, just enough residual birefringence is present in the outer wings of the absorption lines to obtain 90* polarization rotation and little absorption. When placed between crossed polarizers, the result is a pair of narrow passbands to either side of the absorption line position. The disruption of the Faraday filter transmission 241
Volume 96, number 4,5,6
OPTICS COMMUNICATIONS
spectrum by intense beams is due to the vapor absorption lineshape modification. For example, bleaching the vapor has the effect of diminishing the birefringence. Although the sample beam intensity is of the order of the saturation intensity near resonance ( - m W / m m 2), the locked beam is in the wings of the absorption line where absorption is low and the saturation intensity is correspondingly high. Filter spectra did not show evidence of the effects of high intensity at the intensity levels used here. We operated a Cs Faraday filter [7] such that it provided a sharp, < 1 GHz passband of usable transmission for the feedback filtering. The filter transmission spectrum, shown in fig. 3 as trace (a), consisted of passbands on either side of the lines of the Cs 6 zS~/2-6 2p3/2 hyperfine doublet separated by 9.2 GHz. Generally, the shape of the transmission spectrum depends on the cell length, the vapor density and the magnetic field strength. To obtain these sharp, well defined bands, we operated a 2 inch cell at 80°C and 14 G. The filter featured orthogonal Brewster windows for the cell faces, and Glan Thompson polarizers.
3. High resolution line position measurements A heterodyne diagnostic was used to measure the laser line position with an accuracy of about 40 MHz. The diagnostic, shown in fig. 1, produced the typical data shown in fig. 2. Photodiode D 1 detected the beat of the locked laser output against a second calibrated, frequency swept, diode laser. The detector signal was then filtered by a capacitively coupled 30 MHz low pass filter in series with a diode rectifier. The laser sweep was calibrated against its transmission through a reference Faraday filter, detected by photodiode D2. The sharp features of the Faraday filter provided a convenient and precise frequency reference. We estimate that the peaks of the reference filter were stable to within a few tens of MHz, given our ability to control the filter's vapor temperature and magnetic field to better than 1%. Both the reference filter transmission spectrum and the filtered beat signal were simultaneously recorded by a digital oscilloscope. Curves (a) and (b) in fig. 2 were typical measurements of heterodyne diagnostic signal and the reference filter spectrum. Curve (e) 242
(a)
(t) IM
15 February 1993
. . . . . . . . . . . . . . . . . . .
~'[
. . . . .
. _mOt
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1 1 r 1 1 I 1 1 T 1 T Iil
0 FREQUENCY (GHz)
I
I I 1
10
Fig. 2. Heterodyne detected diode laser frequencylocking data with frequencycalibration. Trace (a), the heterodynesignal, was recorded simultaneously with the reference filter transmission trace (b). The calculated transmission, (c), was scaled to (b) to provide a frequencyscale to the heterodynedata. is a theoretical calculation of the reference filter spectrum [ 7 ]. By fitting the sharp features of the reference spectrum to the calculation, we related a frequency scale to the laser frequency sweep. With these calibrations we were able to confidently compare the heterodyne spectrum measurements taken at different times. By adjusting the current, the laser could be made to lock at any of the four sharp peaks of the Faraday filter spectrum. Figure 3 presents a composite of four separate heterodyne measurements along with a transmission measurement of the locking filter. To construct this figure, each of the heterodyne data were calibrated using their associated reference transmission cell spectrum as described above. Each of the locked peaks are separated by about 4 mA of diode current. We observed the sensitivity of the locked laser frequency to pump current using a Fabry-P6rot interferometer with a free spectral range of 1 GHz. Without the optical feedback, we measured a smooth, linear variation in the frequency with current at a rate of 1.1 GHz/mA. In a locked state, the interference rings remained stationary over a range of 1.2 mA of current variation, corresponding to a locking
Volume 96, number 4,5,6
OPTICS COMMUNICATIONS
15 February 1993
(e) ml ,,d < Z
ill I-W a
(d)
O~ W Z >rn
I
I
I
-500
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CC W I-uJ "1-
I
I
I
l
I
I
I
I
I
I
T
I
0 500 FREQUENCY (MHz)
Fig. 4. Hops between adjacent feedback cavity modes. On a fine
frequency scale, (determined from the reference spectrum (c)), we observed mode hops of ~ 230 MHz, corresponding to the 65 cm feedback cavity length.
-5
0
5
FREQUENCY (GHz)
Fig. 3. Effect of diode laser current on locking frequency. Trace (a) shows the transmission spectrum of the locking filter. Traces (b), (c), (d) and (e) show the operating frequency for input currents of 33.79, 37.98, 43.09 and 46.35 mA, respectively. range of 1.3 GHz, a few times larger than the locking ranges of 400 and 300 M H z obtained by refs. [2,4], respectively. Without differentiating between the four possible locking positions, ref. [ 3 ] reported an enormous locking range of 440 G H z for a laser diode in an external cavity locked by an intracavity Faraday filter. We observed that the laser frequency occupied modes formed by the feedback mirror and a laser facet. The separation between the mirror and the laser of 65 cm produced a mode spacing of about 230 MHz, or about 5 modes to a filter passband. Shifts in the mode comb, arising from feedback pathlength changes of ~ 2 / 2 , resulted in a mode hop every few minutes. Figure 4 exhibits the heterodyne signals before and after such a hop. Generally, the output frequency follows the shifting mode until a neighboring mode becomes a stronger source of feedback. Thus,
the locked laser output frequency could be fine tuned by controlling the length of the feedback arm using a piezoelectrically driven mirror. Figure 4 also indicates the ~ 4 0 MHz resolution provided by our heterodyne diagnostic, quoting the resolution as 10% of the signal width (fwhm). As in our previous figure, these signals were obtained in the course of a wide sweep in the frequency of the reference laser across the reference filter spectrum to establish absolute frequency position and scale. The ~ 400 MHz breadth of the signal was due to the low bandwidth of the oscilloscope (which we traded away for gain) and the relatively rapid reference laser frequency sweep. In other heterodyne measurements we were able to increase the spectral resolution of the diagnostic by sweeping more slowly.
4. Conclusion In summary, we have developed a simple, all optical means for matching the frequency of a commercial diode laser to an atomic line filter passband using filtered feedback locking. A Cs Faraday filter with 1 G H z passbands was used to provide the feedback filtering. We observed diode emission in the filter passband with relatively little sensitivity to diode current or temperature. Frequency locking resulted in linewidths of less than 30 MHz bandwidth. The 243
Volume 96, number 4,5,6
OPTICS COMMUNICATIONS
laser was c o n t i n u o u s l y l o c k e d to an external m o d e , a l t h o u g h m o d e h o p p i n g o c c u r r e d as instabilities in the f e e d b a c k p a t h shifted the m o d e c o m b . We obs e r v e d d i o d e e m i s s i o n in the filter p a s s b a n d w i t h relatively little sensitivity to d i o d e current or t e m p e r ature. Because w e a k c o u p l i n g was used, the l o c k e d laser p o w e r was c o m p a r a b l e to that o f the u n l o c k e d laser. T h i s laser source shows p r o m i s e for m a t c h e d optical t r a n s c e i v e r s using F a r a d a y filters in b o t h the optical t r a n s m i t t e r for locking as well as the receiver.
Acknowledgements T h i s research was f u n d e d by S D I O / I S T u n d e r U S A S D C c o n t r a c t D A S G 6 0 - 8 9 - C - 0 1 2 0 . We wish to
244
15 February 1993
t h a n k Dr. K e p i W u and Dr. J o h n J o h n s o n for t h e i r e n c o u r a g e m e n t and support.
References [ 1] For a review of diode laser feedback locking, see C.E. Weiman and L. Holberg, Rev. Sci. Instrum. 62 ( 1991 ) 1. [2] N. Cyr, M. Breton, M. Tetu and S. Theriault, Optics Lett. 16 (1991) 1298. [3] P. Wanninger, E.C. Valdez and T.M. Shay, IEEE Photon. Technol. Lett. 4 (1992) 94. [4] W.D. Lee and J.C. Cambell, Appl. Phys. Lett. 58 ( 1991 ) 995. [5] P.P. Sorokin, J.R. Lankard and V.L. Moruzzi, Appl. Phys. Len. 15 (1969) 179. [6] T. Endo, T. Yabuzaki, M. Kitano, T. Sato and T. Ogawa, IEEE J. Quantum Electron. QE-13 (1977) 866. [ 7 ] J. Menders, K. Benson, S.H. Bloom, C.S. Liu and E. Korevaar, Optics Lett. 16 (1991) 846.
OPTICS C O M MUN ICAT1ONS
Optical feedback locking of a diode laser using a cesium Faraday filter K y u n a m C h o i , J. M e n d e r s , P a u l S e a r c y a n d E r i c K o r e v a a r Thermo Trex Corporation, 9550 Distribution Avenue. San Diego, CA 92121-2305, USA
Received 9 July 1992; revised manuscript received 6 October 1992
We used weak filtered optical feedback to set the output frequency of a commercial single mode diode laser close to the peak of an atomic line filter passband. A Cs Faraday atomic line filter provided ultra-narrow optical feedback passbands near 852 nm. The locked laser power of 34 mW was nearly that of the unlocked laser. In-band laser operation was tolerant of laser diode current and temperature fluctuations and feedback path instabilities, important qualities in the design of matched filtered optical transceivers. We describe the feedback locked laser diode and present locked spectrum measurements. We show how fine tuning over a 100 MHz range is available by feedback mirror positioning.
1. Introduction
Extremely n a r r o w b a n d single m o d e d i o d e lasers have been developed for applications that require matched transceivers, such as remote sensing a n d optical c o m m u n i c a t i o n s . Optical feedback locking, where filtered feedback controls the o u t p u t frequency, is often used to obtain single m o d e operation. F a b r y - P 6 r o t cavities, gratings, birefringent filters and other mechanically tunable n a r r o w b a n d filters are c o m m o n l y used as the feedback filters [ 1 ]. To take advantage o f state-of-the-art n a r r o w b a n d filters (such as atomic line filters) with 1 G H z passbands in the optical receiver, the absolute frequency o f the laser must be set with a precision o f about a h u n d r e d MHz. The mechanically tunable filters m e n t i o n e d above can achieve this degree o f precision only with the a d d e d complexity of a servo-loop referred to a spectroscopic standard. F o r applications free to operate at the p a s s b a n d frequencies of an atomic line filter, all-optical lockers with intrinsic frequency references, such as those based on resonant phase conjugation [2] and resonant magnetooptic effects (refs. [3,4] and this w o r k ) , are advantageous. The F a r a d a y filter, an o p t i m i z e d resonant magneto-optic filter described in more detail below, has been used as an intraeavity element to frequency lock dye lasers to the s o d i u m D lines [5,6]. Re240
cently, a diode laser in an external cavity was frequency locked by an intra-cavity F a r a d a y filter [ 2 ]. In other recent work, a diode laser was locked using a weekly transmitting magneto-optic filter to provide filtered feedback [4], reducing the usable output power. By using the F a r a d a y filter to provide filtered feedback, we show that frequency control can be achieved using the filter outside the cavity without a significant reduction in the usable laser power. In this Letter, we describe our technique for optical feedback locking a single mode commercially available diode laser using a cesium F a r a d a y filter at 852 nm. To show that the locked laser was well m a t c h e d to a F a r a d a y filtered receiver with a gigahertz bandwidth, we measured the laser diode spect r u m with a resolution of about 40 M H z using a calibrated heterodyne diagnostic. The output power of the locked laser diode of 34 m W was comparable to that of the unlocked laser o f 40 mW. We also found that the laser frequency occupied a resonance frequency associated with the cavity formed by the feedback arm and the diode laser. Thus, a fine adj u s t m e n t of the laser frequency can be made by adjusting the feedback arm length.
0030-4018/93/$ 06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
Volume 96, number 4,5,6
OPTICS COMMUNICATIONS
2. Feedback locking technique As shown in fig. 1, feedback locking was accomplished by sampling the laser output, filtering the sampled beam using a Faraday filter, and retro-reflecting the beam into the laser. Using a Faraday filter in conjunction with a laser diode was possible because the weak sampled beam intensity did not disrupt the Faraday filter which can be spoiled by intense beams as described below. Generally, broadband feedback obtained without the filter caused multimode operation of the unlocked diode laser. Under this condition, the laser emission occurred over a band of some twenty modes separated by a mode spacing of about 100 GHz. The band could be temperature a n d / o r current tuned to straddle the Faraday filter passband near 852 nm. The susceptibility of the diode to external feedback proved to be so great, that very weak feedback levels served to control the laser frequency. A feedback level of about ½% was obtained by double passing a 15% beamsplitter and a 50% Faraday filter transmission peak. We had enough excess feedback power to use the Faraday filter without concern for optimizing its transmission, and to make use of a 2 m convex feedback mirror to ease the alignment tolerance. The feedback easily dominated the weak resonance con-
I
ii
o
] ~~~ 0 [?2
iiPY ; ~1~ i FM I:! FEEDBACK LOCKED LASER
HETERODYNE DETECTION
Fig. 1. Schematic of the optical feedback locked laser cavity configuration and the heterodyne detection setup. The symbols are DL: diode laser; Px, Py: x, y polarizers; Ff: Faraday filters; FM: feedback mirror, O1: optical isolator and D 1, D2: photodiodes.
15 February 1993
dition imposed by the low reflectivity of the uncoated diode facets and the laser spectrum jumped to a position inside the 1 GHz Faraday filter passband. Under these feedback conditions, a narrow laser linewidth of ~<30 MHz was measured using the heterodyne diagnostic. With the 15% feedback arm sampling, we obtained an output power of 34 mW independent of whether the feedback was used to lock the laser or blocked. Further optimization of the Faraday filter transmission can be traded for reductions in the feedback arm sampling, increasing the locked laser output. Within the Faraday filter passband, the laser oscillated in a mode of the cavity formed by the diode and the retro-reflector. To assure that the laser oscillated in a single one of these modes, we configured the feedback cavity length to be about 65 cm to obtain a mode spacing of about 230 MHz, which allowed the Faraday filter passband to discriminate between adjacent modes. For these experiments we used a Spectra Diode Labs SDL-5400-GI CW single mode GaA1As diode laser with a specified bandwidth of ~ 15 MHz. This particular laser emitted about 50 mW at 859 nm for a 71 mA current at room temperature. In order to shift the output to the Cs Faraday filter passband near 852 nm, we operated the laser at about - 1.5°C and 56 mA to produce 40 mW (without feedback locking). Since these lasers are supplied with uncoated facets (which provide an outcoupling of about 75%), they are generally very susceptible to feedback. The Faraday filter, a magneto-optic filter, essentially consists of a resonant Faraday rotator sandwiched between two crossed polarizers. The Faraday rotator usually consists of an alkali metal vapor immersed in an axial magnetic field to provide rotary power in the vicinity of strong absorption lines. The magnetic field Zeeman splits the absorption line and the corresponding dispersion in the refractive index into right and left circular components giving circular dichroism and birefringence. At the fields and temperatures we use, just enough residual birefringence is present in the outer wings of the absorption lines to obtain 90* polarization rotation and little absorption. When placed between crossed polarizers, the result is a pair of narrow passbands to either side of the absorption line position. The disruption of the Faraday filter transmission 241
Volume 96, number 4,5,6
OPTICS COMMUNICATIONS
spectrum by intense beams is due to the vapor absorption lineshape modification. For example, bleaching the vapor has the effect of diminishing the birefringence. Although the sample beam intensity is of the order of the saturation intensity near resonance ( - m W / m m 2), the locked beam is in the wings of the absorption line where absorption is low and the saturation intensity is correspondingly high. Filter spectra did not show evidence of the effects of high intensity at the intensity levels used here. We operated a Cs Faraday filter [7] such that it provided a sharp, < 1 GHz passband of usable transmission for the feedback filtering. The filter transmission spectrum, shown in fig. 3 as trace (a), consisted of passbands on either side of the lines of the Cs 6 zS~/2-6 2p3/2 hyperfine doublet separated by 9.2 GHz. Generally, the shape of the transmission spectrum depends on the cell length, the vapor density and the magnetic field strength. To obtain these sharp, well defined bands, we operated a 2 inch cell at 80°C and 14 G. The filter featured orthogonal Brewster windows for the cell faces, and Glan Thompson polarizers.
3. High resolution line position measurements A heterodyne diagnostic was used to measure the laser line position with an accuracy of about 40 MHz. The diagnostic, shown in fig. 1, produced the typical data shown in fig. 2. Photodiode D 1 detected the beat of the locked laser output against a second calibrated, frequency swept, diode laser. The detector signal was then filtered by a capacitively coupled 30 MHz low pass filter in series with a diode rectifier. The laser sweep was calibrated against its transmission through a reference Faraday filter, detected by photodiode D2. The sharp features of the Faraday filter provided a convenient and precise frequency reference. We estimate that the peaks of the reference filter were stable to within a few tens of MHz, given our ability to control the filter's vapor temperature and magnetic field to better than 1%. Both the reference filter transmission spectrum and the filtered beat signal were simultaneously recorded by a digital oscilloscope. Curves (a) and (b) in fig. 2 were typical measurements of heterodyne diagnostic signal and the reference filter spectrum. Curve (e) 242
(a)
(t) IM
15 February 1993
. . . . . . . . . . . . . . . . . . .
~'[
. . . . .
. _mOt
iLL
8
M "1-
/!
(c) 1111111
!
'
/,
/ '\
]
-10
]
/
/
I ~
_
I
1 1 r 1 1 I 1 1 T 1 T Iil
0 FREQUENCY (GHz)
I
I I 1
10
Fig. 2. Heterodyne detected diode laser frequencylocking data with frequencycalibration. Trace (a), the heterodynesignal, was recorded simultaneously with the reference filter transmission trace (b). The calculated transmission, (c), was scaled to (b) to provide a frequencyscale to the heterodynedata. is a theoretical calculation of the reference filter spectrum [ 7 ]. By fitting the sharp features of the reference spectrum to the calculation, we related a frequency scale to the laser frequency sweep. With these calibrations we were able to confidently compare the heterodyne spectrum measurements taken at different times. By adjusting the current, the laser could be made to lock at any of the four sharp peaks of the Faraday filter spectrum. Figure 3 presents a composite of four separate heterodyne measurements along with a transmission measurement of the locking filter. To construct this figure, each of the heterodyne data were calibrated using their associated reference transmission cell spectrum as described above. Each of the locked peaks are separated by about 4 mA of diode current. We observed the sensitivity of the locked laser frequency to pump current using a Fabry-P6rot interferometer with a free spectral range of 1 GHz. Without the optical feedback, we measured a smooth, linear variation in the frequency with current at a rate of 1.1 GHz/mA. In a locked state, the interference rings remained stationary over a range of 1.2 mA of current variation, corresponding to a locking
Volume 96, number 4,5,6
OPTICS COMMUNICATIONS
15 February 1993
(e) ml ,,d < Z
ill I-W a
(d)
O~ W Z >rn
I
I
I
-500
o
CC W I-uJ "1-
I
I
I
l
I
I
I
I
I
I
T
I
0 500 FREQUENCY (MHz)
Fig. 4. Hops between adjacent feedback cavity modes. On a fine
frequency scale, (determined from the reference spectrum (c)), we observed mode hops of ~ 230 MHz, corresponding to the 65 cm feedback cavity length.
-5
0
5
FREQUENCY (GHz)
Fig. 3. Effect of diode laser current on locking frequency. Trace (a) shows the transmission spectrum of the locking filter. Traces (b), (c), (d) and (e) show the operating frequency for input currents of 33.79, 37.98, 43.09 and 46.35 mA, respectively. range of 1.3 GHz, a few times larger than the locking ranges of 400 and 300 M H z obtained by refs. [2,4], respectively. Without differentiating between the four possible locking positions, ref. [ 3 ] reported an enormous locking range of 440 G H z for a laser diode in an external cavity locked by an intracavity Faraday filter. We observed that the laser frequency occupied modes formed by the feedback mirror and a laser facet. The separation between the mirror and the laser of 65 cm produced a mode spacing of about 230 MHz, or about 5 modes to a filter passband. Shifts in the mode comb, arising from feedback pathlength changes of ~ 2 / 2 , resulted in a mode hop every few minutes. Figure 4 exhibits the heterodyne signals before and after such a hop. Generally, the output frequency follows the shifting mode until a neighboring mode becomes a stronger source of feedback. Thus,
the locked laser output frequency could be fine tuned by controlling the length of the feedback arm using a piezoelectrically driven mirror. Figure 4 also indicates the ~ 4 0 MHz resolution provided by our heterodyne diagnostic, quoting the resolution as 10% of the signal width (fwhm). As in our previous figure, these signals were obtained in the course of a wide sweep in the frequency of the reference laser across the reference filter spectrum to establish absolute frequency position and scale. The ~ 400 MHz breadth of the signal was due to the low bandwidth of the oscilloscope (which we traded away for gain) and the relatively rapid reference laser frequency sweep. In other heterodyne measurements we were able to increase the spectral resolution of the diagnostic by sweeping more slowly.
4. Conclusion In summary, we have developed a simple, all optical means for matching the frequency of a commercial diode laser to an atomic line filter passband using filtered feedback locking. A Cs Faraday filter with 1 G H z passbands was used to provide the feedback filtering. We observed diode emission in the filter passband with relatively little sensitivity to diode current or temperature. Frequency locking resulted in linewidths of less than 30 MHz bandwidth. The 243
Volume 96, number 4,5,6
OPTICS COMMUNICATIONS
laser was c o n t i n u o u s l y l o c k e d to an external m o d e , a l t h o u g h m o d e h o p p i n g o c c u r r e d as instabilities in the f e e d b a c k p a t h shifted the m o d e c o m b . We obs e r v e d d i o d e e m i s s i o n in the filter p a s s b a n d w i t h relatively little sensitivity to d i o d e current or t e m p e r ature. Because w e a k c o u p l i n g was used, the l o c k e d laser p o w e r was c o m p a r a b l e to that o f the u n l o c k e d laser. T h i s laser source shows p r o m i s e for m a t c h e d optical t r a n s c e i v e r s using F a r a d a y filters in b o t h the optical t r a n s m i t t e r for locking as well as the receiver.
Acknowledgements T h i s research was f u n d e d by S D I O / I S T u n d e r U S A S D C c o n t r a c t D A S G 6 0 - 8 9 - C - 0 1 2 0 . We wish to
244
15 February 1993
t h a n k Dr. K e p i W u and Dr. J o h n J o h n s o n for t h e i r e n c o u r a g e m e n t and support.
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