Download Characterisation of Extended Cavity alpha

Characterization of Extended Cavity α-Distributed Feedback
Laser at 1062 nm
W A A Syed and Jow-Tsong Shy*
Department of Physics, COMSATS Institute of Information Technology, Islamabad,
Pakistan.
*
Department of Physics, National Tsing Hua University, Hsinchu, Taiwan ROC
[email protected]
Abstract. We report the construction and characterisation of α-Distributed Feedback
laser in compact extended cavity geometry. Up to 650 mW of single frequency power
operating near 1062 nm is generated that is continuously frequency tunable more than
6.5 GHz at constant current and temperature. A partial reflector is employed to
enforce stable single-mode operation in the strong feedback regime. Compared to
other configurations where a grating is used, the lower loss in the extended cavity
allows higher fundamental power available for second harmonic generation for the
useful spectroscopy of iodine spectrum near 531 nm. The performance of this α-DFB
laser is compared to that of visible and UV laser diodes of previous experiments in
extended cavity geometry. Possible spectroscopic applications are discussed
1. Introduction
The basic requirement for high resolution atomic and molecular spectroscopy is a single-mode
continuously tunable laser with sub-megahertz linewidth output. A simple and attractive way of
achieving this with a laser diode is to use a partially reflecting mirror in an extended cavity
configuration. Due to the low cost and long lifetime, diode lasers are particularly attractive for high
resolution spectroscopy. The extended cavity configuration forced the laser to oscillate single mode
[1-3] with acceptably enough tunable range for most of the spectroscopic applications. The laser diode
emitting in visible regions [4-5] have relative low power, limited spectral range and linewidths in tens
of megahertz range. The extended cavity can narrow the linewidth of a laser to less than a megahertz
and provides discontinuous tuning over several gigahertzes. Furthermore, by non-linear frequency
generation we can cover a broader range with high power.
Early available lasers were mostly FP heterostructures, whose frequency control was achieved by
means of frequency selective feedback from grating, etc. Nowadays, diode lasers already tuned at
given atomic resonance are commercially available. The rapid development of Distributed Feedback
(DFB) lasers [4] has led to the recent commercial availability of cw-laser diodes having practical
lifetime characteristics. Such developments for these devices afford exciting possibilities in atomic
physics by offering a low-cost and compact source. A distributed Bragg grating etched onto the active
layer of a semiconductor laser naturally locks the central wavelength within the gain band, and only a
single longitudinal mode profits from the available energy. The optical structure is sensitive to
variations in the refractive index of the active medium due to carrier density and temperature. When
the laser’s current and temperature are accurately controlled, the peak wavelength can be tuned along
an acceptable range. The control through current is fast, but the sensitivity on central frequency is
weak, on the order of 0.01 nm/mA. This sensitivity is weak for a large tuning, but is strong enough
that it should be taken into account to obtain a flat output power while tuning wavelength through
temperature. Controlling wavelength and power can become rather complex when both are functions
of forward current and laser temperature.
Obtaining the desired stability in DFB structure i.e. slow-rate central wavelength, requires
thermoelectric cooling and temperature control. Since, the tuning mechanism based on temperature
control affects the output power, feedback control is required. Temperature also affects the wavelength
and the device’s quantum efficiency. The application of a correction factor to the laser current to
maintain power at a fairly stable level throughout the range also shifts the wavelength. The carrier
density must be taken into account, since, a change in the laser current at the semiconductor junction
affects the thermal equilibrium of the material. The stability in power and wavelength which
contributes to the error in measurements provides the ideal source for long-term environmental testing.
The fact that these sources can now provide a high output power with such excellent power stability is
a great advantage. Precise control of temperature and power parameters is relatively easy for the
integrated DFB laser. A careful setting of these parameters allows the DFB laser to be tuned over a
wide range of wavelengths, while maintaining a high degree of wavelength and power stability.
Frequency-stable operation of an extended-cavity with GaN diode laser are reported [6-7,12]. In
order to obtain radiation in the visible range from a commercial IR diode laser, it is essential to double
its frequency by placing a nonlinear crystal in an enhancement cavity [6,8-9]. Conroy et al. [1]
characterized their laboratory-made ECDL and determined the upper limit of the linewidth of 5 MHz.
Hayasaka et al. [10] measured an instantaneous linewidth of 2 MHz of laboratory-made ECDL. The
use of this extended-cavity diode lasers (ECDLs) is in precise and high-resolution spectroscopy, lasercooling and optical frequency measurements [3].
2. Experimental Setup and Discussion
In the present work we describe an alternative configuration for an extended cavity, consisting of a
partially reflecting meniscus lens attached with PZT and α-Distributed Feedback laser operating near
1062 nm. The advantages of α-DFB laser include, frequency stability over a longer period, reduced
sensitivity to optical feedback, and temperature tuning of the frequency without hops. Our αDistributed Feedback (SDL-6752 P1) operates at 1062.29-1062.60 nm with 850 mW at an operating
current of 3.5 amperes. Its output frequency is tunable over 400 GHz by changing the injection current
and temperature. The current and temperature coefficients are -96 MHz/mA and -24 GHz/K
respectively. The output beam has a circular profile with only 6 mrad FWHM divergence angle;
hence, no collimating lens is used in our experiment. The laser package includes an internal
thermoelectric cooler (peltier) and a thermistor for temperature control. The laser usually operates in
single longitudinal-mode regime at room temperature with a linewidth of about 2 MHz. In the absence
of an optical feedback, the linewidth of the DFB laser is comparable to the FP laser. The criteria of the
line narrowing are same for both FP and DFB lasers. Since the grating grown is directly in the area of
the active medium of the DFB laser, the external cavity can be closed with the presence of simple
mirror.
Fig. 1 shows a diagram of the extended cavity setup. The laser and the partially reflecting meniscus
lens attached with the piezoelectric transducer is mounted on Lee’s mirror mount, which is one of the
best available mount with extremely fine 3-d tunability, and is automatically centered with respect to
the accompanying aspheric collimating optic. The mirror mount allows the control of its tilt angle for
maximizing the laser feedback and laser output. The Piezo-tube allows tuning of the laser for the
required frequency. All these are placed on a heavy thick aluminum block inside a closed box with
passive insulation to reduce acoustical noise coupling. For the same reason, the laser exit is closed
with a glass window coated for 1062 nm. The length of the extended cavity is 75 mm. This is a lowcost, compact, simple geometry which can offer high performance [21] mainly due to the stability and
construction of the mirror mount. For further stability in operation, we used a heavy single piece
aluminum block, where both laser and mount are fixed. Furthermore, a special purpose rubber sheet,
which is used as vibration-isolator, is placed in between the optical table and the aluminum block. For
high power diode lasers (>30 mW), the output facet has sufficiently low reflectivity [11], so that
antireflection coatings are not necessary. Therefore, our setup seems particularly well-suited for these
high power lasers. The overall volume of the system was not more than one-fourth of cubic feet. Fig. 2
shows the resonance peaks of a home-made scanning Fabry–Perot interferometer (FSR = 1.5 GHz) for
the laser, both with and without optical feedback.
We have assessed the performance of the EC-DFB laser with reference to the laser with out an
optical feedback. The inner surface of the meniscus lens has a radius of curvature of 10 cm with a
partial reflectivity of 30%; making a stable cavity. On the other hand, the outer surface of the lens has
a radius of curvature of 3.3 cm with anti-reflection coating at 1062 nm. The reflected component
provides the feedback necessary for the extended cavity. The output is then passed through the
collimating lens and a Faraday optical isolator is placed to completely isolate it with the rest of the
components. It is to be expected that the extended cavity having meniscus lens would give higher
slope efficiency with a wider tuning range. From Fig. 3 it can be seen that the threshold current was
much lower with larger slope efficiency with the optical feedback as compared to the same without
optical feedback. The difference between the laser output with and without the optical feedback is
found to decrease with increasing current.
Compositional fluctuations within the wells cause locally-enhanced carrier recombination and
increase in the inhomogeneous spontaneous emission linewidth below the lasing threshold. Particular
spatial regions therefore contribute to specific spectral regions. During tuning, the different spectral
regions become dominant in turn and this produces the observed jumps. Confirmation of the existence
of these compositional fluctuations in InGaN laser is discussed in reference [12-13]. These
compositional fluctuations are believed to limit the discontinuous tuning of the violet ECDL [1] and
give rise to discontinuities in the gain spectrum causing wavelength jumps in the coarse tuning of the
ECDL. Wavelength jumps of about 0.4 nm have been reported by Nakamura et al. [12] for current
tuning of the single-mode emission of free-running InGaN MQW laser. Fortunately, these strong
compositional fluctuations do not arise in near infrared and red diode lasers. Conroy et al. [1] have
operated a compact ECDL with both 635 and 670 nm laser diodes for comparison, and found that
tuning up to 10 nm is possible.
Our EC-DFB laser operates in Littrow geometry [14 ] and gives a tunable, single frequency output
in excess of 650 mW. The output could be tuned smoothly and continuously over 6.5 GHz, and an
upper limit of 1 MHz was placed on the linewidth. The threshold current and the slope efficiency are
approx. 0.5 amperes and 0.15 mW/mA respectively without optical feedback; whereas the same are
less than 0.3 amp and 0.18 mW/mA respectively with optical feedback. This type of laser is normally
operated far above threshold and has, for instance, been used in experiments demonstrating amplitude
squeezing at room temperature [18]. Manoel et al [15-16] have used the same type of device to
decelerate a calcium atomic beam. Further to this we have obtained the second harmonics by using a
periodically poled LiNbO3 crystal, showing that enough light can be generated from a single-diode
laser to be used for example in the frequency modulation spectroscopy of the iodine molecule near 531
nm. The performance of DFB laser is compared with and without extended cavity. We find important
differences that need to be taken into consideration by spectroscopic users. To the best of our
knowledge this is the first in-depth study of the performance of DFB laser with extended cavity
geometry.
The α-DFB laser is used in Littman configuration [17] to assure stable single-mode operation and
tuning. This is particularly important for second harmonic generation, where efficiency is proportional
to the square of the fundamental power, and may also be potentially useful for squeezing experiments
[18]. The Littman and Littrow [14,17] schemes, are well-known and widely used to enforce controlled
single-mode operation in diode lasers. However, very often the scattering or absorption losses can be
quite large (about 30 to 40 percent), and thus the price to achieve single-mode operation is reduced
power. However, this loss can be greatly reduced by using a proper selection of low-loss optics for a
particular wavelength. When power is relevant, a usual solution is to injection-lock a second, highpower device [19]. For example, in trapping alkaline-earth elements, tens of milliwatts of single-mode
radiation are needed in the blue-violet region, which can be achieved by second harmonic generation
of near infrared diode lasers [20]. Clifford et al. [21] have presented violet diode laser placed in an
extended cavity configuration in the Littrow geometry. In order to have more available power at the
second harmonics, proportional to the square of the fundamental power, we have built the extended
cavity in an alternative way.
A simple manually set voltage divider is used for tuning the frequency without hop. Higher optical
feedback (above 30%) may possibly produce better results, but we have limited ourselves to 30%
optical feedback. The output could also be continuously tuned by means of a low voltage PZT element
used to change the extended cavity length. For a voltage of 10 V peak-to-peak, the output could be
tuned over 6.5 GHz without mode-hopping and this is sufficient for many high-resolution
spectroscopic purposes. With larger PZT extension and injection current tuning, greater continuous
tuning ranges should be possible. A fine setting enables us to increase the tuning without hop for
infinite long (i.e. beyond which the diode no longer operates in a single axial mode). A homemade
scanning etalon is used to determine the laser linewidth and tuning range without mode hop. It is
believed that the instantaneous linewidth of the laser is in the region of 1 MHz.
For a fixed current and without optical feedback, the laser wavelength can be tuned by controlling
the temperature. In our extended cavity setup, we have 3 frequency-selective components: the laser
cavity (with modes separated by 40 GHz), the plate (etalon modes separated by 620 GHz), and the
extended cavity (73 mm). The wavelength can be tuned either by changing the injection current or by
changing the voltage to PZT. The laser jumps between the modes of the laser cavity (separated by 40
GHz or 0.1 nm), or between the modes of the extended cavity (separated by 620 GHz or 1.5 nm).
Between these larger wavelength jumps, continuous and modehop-free tuning is observed in intervals
of 1.5 GHz, which corresponds nearly to the spacing between the extended cavity mode. No attempt
was made to increase this range. However, a significant improvement could be expected with the
proper combination of tilting mirror and current tuning; or by combining the tilting and translation of
the plate.
Clifford et al. [21] have measured the spectral distribution of amplitude modulation (AM) and
frequency modulation (FM) noise. The sum corresponds to the laser noise and the difference to the
shot noise. For the FM noise measurements, the same setup is employed, but a low finesse home-made
Fabry–Perot cavity is used as a frequency-to-amplitude converter. Under optical feedback, the AM
noise is reduced and comes very close to the shot noise. This same type of laser has been used with
optical feedback and pump suppression techniques [22 ] to achieve AM noise reduction below the shot
noise level (amplitude squeezing) [18]. The extended cavity that we describe here may be
advantageous for these types of experiments. However, the general applications of this configuration
to other high power diode lasers still need to be verified; especially for devices which are naturally
multi-mode.
Iodine-stabilized green laser (543 nm He-Ne), frequency-doubled 1064 nm Nd:YAG, etc. show
better performance as compared to the commonly used iodine-stabilized 633 nm He-Ne laser systems.
Therefore, we have chosen the option of frequency doubling by 0.5 mm thick and 50 mm long PPLN
crystal with AR coated facets. High-conversion efficiencies have been demonstrated for frequencydoubling diode lasers with this crystal. We have used temperature controllers with stability better than
0.01 K. The temperature controller allows the adjustment of the rate of the temperature change. This is
particularly important for slowly tuning the PPLN crystal to the quasi-phase matching temperature of
160°C. The EC-DFB can make available at least 10 mW of single-mode green light for second
harmonic generation. However, our frequency-doubling setup has losses in some optical components,
which limit the power available for doubling. The glass exit window of the box is not AR-coated and
has loss of about 10%. In addition, the optical isolators account for a total loss of 15%. This is
necessary to avoid feedback from the doubling cavity. As a consequence of these losses, we have 450
mW at 1062 nm available for frequency-doubling. For FM-spectroscopy of iodine molecule, 5-10 mW
of green light is sufficient.
3. Conclusion
From the probing of atmospheric gases to solid state materials, the narrow emission linewidth and the
relatively high-power characteristics of tunable light source, is essential to spectroscopic
measurements. We have described an alternative extended cavity setup with reduced losses, which
seems to be particularly suitable for use with high-power diode lasers and for second harmonic
generation. α-DFB diode laser is employed with a partially reflecting meniscus lens to provide about
30% strong optical feedback. About 650 mW of single-mode power in the infrared has been obtained
with continuous tunability over 6.5 GHz. An instrument-limited linewidth less than 1 MHz is
estimated; though it is believed that the actual linewidth is substantially below than this value.
Considerable reduction of the laser linewidth and the frequency noise, under optical feedback is
observed. In an optimized frequency-doubling setup, using periodically poled LiNbO3, about 10 mW
of green power near 531 nm has been generated from 650 mW of infrared power. By reducing losses
between the laser and the doubling cavity, this extended cavity diode laser can be used for high
resolution spectroscopic purposes for iodine molecule near 531nm.
Figure 1 Extended cavity setup; the 75 mm long cavity formed by DFB laser and partially reflecting
meniscus lens is fixed on a heavy aluminium block.
Figure 2 The resonance peaks of a home-made scanning Fabry–Perot interferometer (FSR = 1.5 GHz) for
the laser, both with and without optical feedback.
Light output (mW)
Figure 3 The output powers with and without optical feedback; threshold current was much lower with
larger slope efficiency with the optical feedback as compared to the same without optical feedback and the
difference between the laser output with and without the optical feedback is found to decrease with
increasing current.
Without Feedback
With Feedback
Current (amp)
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
Conroy R S, Hewett J J, Lancaster G P T, Sibbett W, Allen J W, Dholakia K 2000 Opt. Commun. 175 185
Camparo J C 1985 Contemp. Phys. 26 443
Wieman C E, Hollberg L 1991 Rev. Sci. Instrum. 62 1
Nakamura S, Kaenders W April 1999 Laser Focus World, April
Johnson N M, Nurmikko A V, DenBaars S P 2000 Phys.Today 53 31
Hayasaka K 2002 Optics Communications 206 401
Leinen H, Gl€aßner D, Metcalf H, Wynands R, Haubrich D, Meschede D 2000 Appl. Phys. B70 561
Hemmerich A, McIntyre D H, Zimmermann C, Hansch T W 1990 Opt. Lett. 15 372
Hayasaka K, Watanabe M, Imajo H, Urabe S 1994 Appl.Opt. 33 2290
Hayasaka K, Urabe S, Watanabe M 2000 Jpn. J. Appl. Phys.39 L687
Tkach R W, Chraplyvy A R 1986 J. Lightwave Technol. LT-4 1655
Nakamura S, Fasol G, Gill 1997 The Blue Laser Diode, Springer- Verlag 308
Kisielowski C 1998, paper We-01, 2nd International Symposium on Blue Laser and Light Emitting
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
Diodes, Chiba, Japan
Demtroder W 1996 Laser Spectroscopy, Springer, Berlin p116
Manoel D A, Cavasso-Filho R L, Scalabrin A, Pereira D, Cruz F C, 2002 Optics Commun 201 157
Woehl Jr. G, Garcia G A, Cruz F C, Pereira D, Scalabrin A 1999 Appl. Opt. 38 2540
Littman M G, Metcalf H J 1978 Appl. Opt. 17 2224
Zhang T C, Poizat J Ph., Grelu P, Roch J F, Grangier P, Marin F, Bramati A, Jost V, Levenson M D,
Giacobino E 1995 Quant. Semiclass. Opt. 7 601
Marquardt J H, Cruz F C, Stephens M, Oates C, Hollberg L W, Bergquist J C, Welch D F, Mehuys D
1996 Proc. SPIE Conf. 34 2834
Oates C W, Bondu F, Fox R, Hollberg L 1999 Eur. Phys. J D7 449
Clifford M A, Arlt J, Courtial J, Dholakia K 1998 Opt. Commun. 156 300
Machida S, Yamamoto Y, Itaya Y 1987 Phys. Rev. Lett. 58 1000