An all-optical locking of a semiconductor
laser to the atomic resonance line with 1
MHz accuracy
Xiaogang Zhang,1 Zhiming Tao,1 Chuanwen Zhu,1 Yelong Hong,1 Wei
Zhuang,1,∗ and Jingbiao Chen1,2
1 State
Key Laboratory of Advanced Optical Communication Systems and Networks, Institute
of Quantum Electronics, School of Electronics Engineering & Computer Science, Peking
University, Beijing 100871, China
2 [email protected]
∗ [email protected]
Abstract:
An all-optical locking technique without extra electrical
feedback control system for a semiconductor laser has been used in
stabilizing the laser frequency to a hyperfine crossover transition of
87 Rb 52 S
2
1/2 , F = 2 → 5 P3/2 , F = 2, 3 with 1 MHz level accuracy.
The optical feedback signal is generated from the narrow-band Faraday
anomalous dispersion optical filter (FADOF) with nonlinear saturation
effect. The peak transmission of the narrow-band FADOF corresponding to
52 S1/2 , F = 2 → 52 P3/2 , F = 2, 3 crossover transition is 18.6 %. The bandwidth is as wide as 38.9 MHz as the laser frequency changes. After locking,
the laser frequency fluctuation is reduced to 1.7 MHz. The all-optical laser
locking technique can be improved to much higher accuracy with increased
external cavity length. The laser we have realized can provide light exactly
resonant with atomic transitions used for other atom-light interaction
experiments.
© 2013 Optical Society of America
OCIS codes: (140.2020) Diode lasers; (120.2440) Filters; (230.2240) Faraday effect.
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1.
Introduction
Since the first semiconductor laser diode came out in 1962, the significant development has occurred in the field of semiconductor laser. Because of their high efficiency, small size, low cost
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18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028010 | OPTICS EXPRESS 28011
and tunability, semiconductor lasers are now used in various applications, such as optical communication[1], optical sensing systems[2], laser cooling[3] and atomic frequency standard[4-5].
However, in these applications, narrow linewidth and frequency-stabilized semiconductor lasers
are needed. Normally, the emission linewidth of a bare laser diode is typically between 10-100
MHz. In order to reduce the laser diode emission linewidth, an external cavity has been coupled
to the diode laser. With this method, The linewidth is narrowed and the frequency tuning of the
doide laser is improved. Common external cavity diode lasers (ECDLs) in either Littrow or
Littman-Metcalf configurations use diffractive gratings for frequency selection[6-7]. But these
lasers need precise alignment and are very sensitive to the acoustic and mechanical vibrations,
so additional frequency locking techniques should be used to stabilize laser frequency fluctuations. Besides, The laser frequency need to be stabilized to the atomic resonance line in atomic
spectra experiments[8-9].
Here, we demonstrate a novel narrow-band all-optical locking technique to stabilize a semiconductor laser frequency by utilizing narrow-band Faraday anomalous dispersion optical filter
(FADOF) with nonlinear saturation effect of a Rubidium(87 Rb) atomic vapor operating at 780
nm. In this technique, the light possessing the information of the narrow-band FADOF atomic
resonance spectral profile is injected into the laser. As we know, FADOF has advantages of high
transmission, high noise rejection[10] and narrow bandwidth[11] and is critically important in
fields as diverse as free space optical communication, lidar remote sensing systems[12-17] and
so on . Besides, FADOFs are also very potential in the frequency stabilization field of ECDLs,
which makes the laser immune to fluctuations of current and temperature [18-19]. However,
because the transmission band of normal FADOFs in line-center opration is so wide, the laser
frequency of ECDLs with normal FADOFs stabilizing[18-19] can’t be locked to the exactly
atomic transition line since there is GHz level detuning from the transmission center to atomic
lines.
In order to realize the narrow-band all-optical locking to the atomic resonance line exactly, FADOFs require not only the transmission spectroscopy resonant corresponding to the
atomic transition[20-36], but also a bandwidth as narrow as possible, for example, a bandwidth approaching the natural linewidth at MHz level [37]. In this paper, a narrow-band nonlinear 87 Rb FADOF with a bandwidth of 38.9 MHz and its transmission peak resonant with
52 S1/2 , F = 2 → 52 P3/2 , F = 2, 3 crossover transition at 780 nm has been realized. Then, the
narrow-band FADOF with nonlinear saturation effect is used as a frequency discriminator to
lock the laser frequency to the atomic transition through all-optical feedback. The ultimate
stabilizing results are presented. Finally, we obtain a compact and stabilized laser working at
atomic resonance line. Without the complex electrical feedback control system, this all-optical
locking technique has advantages of accuracy, reproducibility and compactness.
2.
Experimental schematics
Figure 1 shows relevant energy levels of 87 Rb. The natural linewidth of 52 P3/2 is 6 MHz. The experimental setup is shown in Fig. 2. A 5 cm long 96.5 % enriched isotope 87 Rb cell is mounted
in a magnetic shield box to reduce the interference of the earth magnetic field. The internal
magnetic field is produced by permanent magnets and different magnetic field intensities are
obtained by changing the distance of magnets. In the atom-light interaction area, the inhomogeneity of the magnetic field intensity can be ignored. The temperature of the 87 Rb cell is
controlled by temperature control system which consists of a heating wire with precision of
0.2 ◦ C. ECDL represents a 780 nm external cavity diode laser which can be tuned to cover all
of the 87 Rb 52 S1/2 → 52 P3/2 transitions. We adopt EYP-RWE-0790-04000-0750-SOT01-0000
790 nm semiconductor LD. The collimation lens is 4.51 mm and the grating is 1800 lines/mm.
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Received 5 Sep 2013; revised 24 Oct 2013; accepted 29 Oct 2013; published 7 Nov 2013
(C) 2013 OSA
18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028010 | OPTICS EXPRESS 28012
Fig. 1. Relevant energy levels of 87 Rb.
The laser beam from 780 nm ECDL is evenly divided into two parts by a beam splitter (BS1 ).
One is used for nonlinear saturated absorption spectra (SAS) as a standard frequency reference. The reference saturated absorption spectra of 87 Rb is collected by a photodiode (PD1 ).
The other is injected into the narrow-band FADOF system. In this system, the laser beam is
divided into two parts by a polarization beam splitter (PBS). Through rotating the half-wave
plate (HWP) before the PBS, we can change the intensity ratio between the two laser beams.
Then, we use the strong one(1.28 mW) as a pumping laser while the weak one(0.58 mW) as a
probe laser. The angle between pumping and probe beams is so small that the residual Doppler
broadening is negligible.
The narrow-band FADOF with nonlinear saturation effect are realized by the following process. The polarization of the probe beam is changed by passing through the 87 Rb cell due to
the circular dichroism induced in the 87 Rb vapor by a nonlinear interaction with the strong
circular polarization pump beam. Then, the rotated probe beam would pass through a pair of
crossed Glan-Taylor prisms (G1 and G2 ) and is reflected by a mirror (M) with the transmission 20 %. For optical feedback, the pair of crossed Glan-Taylor prisms serves as an analyzer.
The extinction ratio of the pair of crossed Glan-Taylor prisms is 1 × 10−5 . Similar to the polarization spectroscopy, the Doppler-free narrow-band transmitted spectroscopy can be obtained.
The transmission of FADOF is measured as the ratio of the maximum transmitted laser power
when the two polarizers are perpendicular to the maximum transmitted laser power when the
two polarizers are parallel. Finally, the optical feedback path is composed of M, G2 , BS2 , 87 Rb
cell, G1 , R1 , PBS, HWP, BS1 and ECDL.
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(C) 2013 OSA
18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028010 | OPTICS EXPRESS 28013
Fig. 2. The all-optical locking experimental setup. ECDL, 780 nm external cavity diode
laser; BS, beam splitter; PBS, polarization beam splitter; G, Glan-Taylor prism; R, high
reflection mirror for 780 nm; HWP, half-wave plate; QWP, quarter-wave plate; M, mirror,
the transmission is 20 %; PD, photodiode. SAS, saturated absorption spectra.
3.
3.1.
Experimental results and discussion
Transmitted spectra of narrow-band FADOF with nonlinear saturation effect corresponding to atomic resonance line
Figure 3(a) shows the transmitted spectra of the narrow-band FADOF with nonlinear saturation
effect at 52 S1/2 , F = 2 → 52 P3/2 , F = 1, 2, 3 transitions. The spectra signal is collected by PD2
by sweeping the laser frequency. The laser frequency is changed by sweeping the voltage of
PZT. The magnetic field intensity is set to 11 G and the temperature of 87 Rb cell is 85 ◦ C. Here,
the optimized magnetic field and temperature has been chosen to maximize the transmitted
spectra. The maximum transmission is 18.4 % at the crossover transition F = 2 → F = 2, 3.
In Fig. 3(a), the reflected probe beam is not injected into the laser, i.e., the laser is in the freerunning condition.
The labels in various peaks in Fig. 3(a) is determined by the upper saturated absorption
spectra of 87 Rb. As we know, The six peaks from left to right of the saturated absorption spectra
of 87 Rb correspond to transitions of F = 2 → F = 1, F = 2 → F = 1, 2, F = 2 → F = 1, 3,
F = 2 → F = 2, F = 2 → F = 2, 3, F = 2 → F = 3, respectively. Spectra lines F = 2 →
F = 1, 2, F = 2 → F = 1, 3 and F = 2 → F = 2, 3 correspond to the crossover transitions.
The bandwidth of the spectral profile is also determined through the frequency interval between
transitions F = 2 → F = 2 and F = 2 → F = 3, which is 266.65 MHz. Comparing to the SAS,
the bandwidth of 87 Rb spectral profile is 34.9 MHz at the crossover transition F = 2 → F = 2, 3
in Fig. 3(a). It’s much narrower than the normal FADOF[20-34].
Since the maximum spectral peak exhibits at crossover transition F = 2 → F = 2, 3, stable
all-optical feedback locking is achieved by injecting the reflected probe beam into the laser
through a mirror(M). As a result, the laser frequency is locked to the crossover transition F =
2 → F = 2, 3. Figure 3(b) shows the transmitted spectra of the narrow-band FADOF with
nonlinear saturation effect measured as a result of the injection, i.e., the laser is in the all-optical
feedback condition. The optical path length between the laser diode and the mirror M is 1.2 m.
The transmission of the peak corresponding to the crossover transition F = 2 → F = 2, 3 is
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Received 5 Sep 2013; revised 24 Oct 2013; accepted 29 Oct 2013; published 7 Nov 2013
(C) 2013 OSA
18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028010 | OPTICS EXPRESS 28014
Fig. 3. The transmitted spectral profile of 87 Rb 52 S1/2 , F = 2 → 52 P3/2 , F = 2, 3 crossover
transition with magnetic field intensity of 11 G and temperature of 85 ◦ C. (a) The transmitted spectra of the narrow-band 87 Rb FADOF with nonlinear saturation effect by using
a free-running laser. The upper line is saturated absorption spectra of 87 Rb and the bottom
line is the transmitted spectra of the narrow-band 87 Rb FADOF. (b) The profile measured
by injecting the reflected probe beam into the laser. The upper line is the saturated absorption spectra of 87 Rb and the bottom line is the transmitted spectra of 87 Rb FADOF with
nonlinear saturation effect by injecting the reflected probe beam into the laser. The optical
path length between the laser diode and the mirror M is 1.2 m.
17.5 %. The flat segments of the spectra at F = 2 → F = 2, 3 transition indicate the amount of
the transmission remains nearly constant. Hence, the laser’s frequency is virtually unchanged.
The bandwidth is as wide as 44.4 MHz at F = 2 → F = 2, 3 crossover transition as the laser
frequency changes. Because the high reproducible atomic resonance line, the laser frequency
can be fixed to the atomic resonance line every time the laser is locked.
The same result at the crossover transition F = 2 → F = 1, 3 is also achieved at the same
experimental conditions. As is shown in Fig. 4(b), The transmission of the peak corresponding
to the crossover transition F = 2 → F = 1, 3 is 20 % and the bandwidth is as wide as 46.4 MHz
as the laser frequency changes.
3.2.
Frequency locking results with all-optical locking technique at different cavity lengths
In all-optical locking scheme, the external cavity length is the optical path length between the
laser diode and the mirror M. Different external cavity lengths present different distributions
of cavity modes. When the laser frequency coincides with a longitudinal mode of the cavity
resonator, the laser frequency is automatically locked to the longitudinal mode while exhibiting
a dramatic reducing in fluctuation of the laser frequency. Because of controlling the quantity of
longitudinal modes inside the laser spectra, the free spectral range (FSR) of the cavity modes
extremely influences the stabilized laser frequency. Besides, the FSR becomes smaller as the
extension of the external cavity. Frequency locking results with all-optical locking technique at
different external cavity lengths are presented in the following.
The estimation of the laser frequency accuracy of the all-optical locking can be carried out
by the fluctuation of the laser frequency after all-optical locking. Firstly, the locking results of
the 1.2 m external cavity length are shown in Fig. 5. the upper line is the transmitted spectra
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Received 5 Sep 2013; revised 24 Oct 2013; accepted 29 Oct 2013; published 7 Nov 2013
(C) 2013 OSA
18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028010 | OPTICS EXPRESS 28015
Fig. 4. The transmitted spectral profile of 87 Rb 52 S1/2 , F = 2 → 52 P3/2 , F = 1, 3 crossover
transition with magnetic field intensity of 11 G and temperature of 85 ◦ C. (a) The transmitted spectra of the narrow-band 87 Rb FADOF with nonlinear saturation effect by using
a free-running laser. The upper line is saturated absorption spectra of 87 Rb and the bottom
line is the transmitted spectra of the narrow-band 87 Rb FADOF. (b) The profile measured
by injecting the reflected probe beam into the laser. The upper line is the saturated absorption spectra of 87 Rb and the bottom line is the transmitted spectra of 87 Rb FADOF with
nonlinear saturation effect by injecting the reflected probe beam into the laser. The optical
path length between the laser diode and the mirror M is 1.2 m.
of narrow-band 87 Rb FADOF with nonlinear saturation effect by injecting the reflected probe
beam into the laser. The locking result is presented as the laser frequency is scanned across
the atomic resonances. The peak transmission corresponding to F = 2 → F = 2, 3 crossover
transition is 17.5 %. The bandwidth is as wide as 44.4 MHz as the laser frequency changes. The
bottom line is the frequency locking spectra via optical feedback without frequency scanning.
Transmission of the feedback light is exactly 17.5 %, which indicates that we have succeeded
in locking the laser frequency to a hyperfine crossover transition of the 87 Rb F = 2 → F = 2, 3
by using an all-optical feedback locking technique for a semiconductor laser. The standard deviation of the laser frequency fluctuation is 3.3 MHz corresponding that the standard deviation
of the transmission fluctuation is 1.3 % while there is 17.5 % variation in the range of 44.4
MHz bandwidth.
As the FSR is 125 MHz for the 1.2 m cavity length, which is much greater than the bandwidth of the narrow-band FADOF transmission spectra 44.4 MHz, the longitudinal mode of
the cavity resonator does not always coincide with the narrow-band FADOF transmission spectra, which induces the instability of the laser frequency. In order to achieve much more stable
laser frequency, the external cavity length should be extended, which means the FSR of the
cavity resonator becomes smaller. In principle, the stabilized laser frequency would be more
stable while the FSR is less than the locking range of the FADOF transmission spectra. Hence,
Considering our conditions, the external cavity length should be longer than 3.3 m at least.
Because of the limitation of the system volume, we extend the cavity length to 6 m. Figure 6
shows the frequency locking results of the 6 m cavity length. The upper line is the transmitted
spectra of 87 Rb FADOF with nonlinear saturation effect by injecting the reflected probe beam
into the laser. The peak transmission corresponding to F = 2 → F = 2, 3 crossover transition is
18.6 %. The bandwidth is as wide as 38.9 MHz as the laser frequency changes. The bottom line
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(C) 2013 OSA
18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028010 | OPTICS EXPRESS 28016
Fig. 5. Frequency locking of the laser at 87 Rb 52 S1/2 , F = 2 → 52 P3/2 , F = 2, 3 crossover
transition with 1.2 m external cavity length with all-optical feedback technique. The upper line is the transmitted spectra of 87 Rb FADOF with nonlinear saturation effect at
52 S1/2 , F = 2 → 52 P3/2 , F = 2, 3 crossover transition by injecting the reflected probe beam
into the laser. The bottom line is frequency locking spectra via optical feedback.
is the frequency locking spectra via optical feedback without frequency scanning. Comparing
with Fig. 5, the standard deviation of the laser frequency fluctuation is 1.7 MHz corresponding
that the standard deviation of the transmission fluctuation is 0.79 %. It can be seen that the
frequency locking effect of 6 m cavity length is much better than that of 1.2 m cavity length. The
reason is that the FSR for 6 m cavity length is only 25 MHz, which is less than the bandwidth of
the narrow-band FADOF transmission spectra, 38.9 MHz. The longer external cavity ensures
that there is always a longitudinal mode of the cavity resonator coinciding with the narrow-band
FADOF transmission spectra. Hence, the laser frequency can be locked within atomic resonance
line by the transmitted spectra of narrow-band 87 Rb FADOF under different conditions. In this
case, extra electrical feedback control system, which is required for stable optical feedback by
controlling the external cavity length between the laser and the mirror M, doesn’t need. Finally,
the all-optical laser locking technique can be improved to much higher accuracy with increased
cavity length.
4.
Conclusion
In conclusion, we have succeeded in stabilizing the laser frequency to a hyperfine crossover
transition of the 87 Rb F = 2 → F = 2, 3 by using an all-optical locking technique without extra
electrical feedback control system for a semiconductor laser at 1 MHz level accuracy. For the
narrow-band FADOF with nonlinear saturation effect, The peak transmission corresponding to
F = 2 → F = 2, 3 crossover transition is 18.6 %. The bandwidth is as wide as 38.9 MHz as
the laser frequency changes. After locking the laser frequency, the laser frequency fluctuation
is reduced to 1.7 MHz. The ECDL we have realized can provide light exactly resonant with
atomic resonance transitions used for other atom-light interaction experiments.
In the future, the all-optical laser locking technique can be improved to much higher accuracy
with increased external cavity length. An 100 m fiber external cavity is under developing to
improve the absolute frequency accuracy of all optical laser locking to atomic resonance line.
With the technique of averaging over fast scanning, kHz accuracy with all optical locking can
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18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028010 | OPTICS EXPRESS 28017
Fig. 6. Frequency locking of the laser at 87 Rb 52 S1/2 , F = 2 → 52 P3/2 , F = 2, 3 crossover
transition with 6 m external cavity length with all-optical feedback technique. The upper line is the transmitted spectra of 87 Rb FADOF with nonlinear saturation effect at
52 S1/2 , F = 2 → 52 P3/2 , F = 2, 3 crossover transition by injecting the reflected probe beam
into the laser. The bottom line is frequency locking spectra via optical feedback.
be expected. The accuracy, reproducibility and compactness of the whole system are promising.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant
Nos.10874009 and 11074011).
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Received 5 Sep 2013; revised 24 Oct 2013; accepted 29 Oct 2013; published 7 Nov 2013
(C) 2013 OSA
18 November 2013 | Vol. 21, No. 23 | DOI:10.1364/OE.21.028010 | OPTICS EXPRESS 28018