WO2014198707A1

WO2014198707A1 - Narrow linewidth semiconductor laser and method

Info

Publication number: WO2014198707A1
Application number: PCT/EP2014/062000
Authority: WO
Inventors: Stéphane SCHILT; Thomas SÜDMEYER; Lionel TOMBEZ
Original assignee: University Of Neuchatel
Priority date: 2013-06-10
Filing date: 2014-06-10
Publication date: 2014-12-18

Abstract

A method for reducing the frequency fluctuations of a semiconductor laser, thus lowering its linewidth, using a high frequency feedback loop that uses the electrical fluctuations measured across the laser as an error signal to stabilize the internal temperature of the laser, and the corresponding narrow-linewidth laser source.

Description

Narrow Linewidth Semiconductor Laser and Method

Field of the invention

[0001] This invention relates to a method for reducing the frequency fluctuations of a semiconductor laser, thus lowering its linewidth, using the electrical fluctuations measured across the laser as an error signal in a feedback loop that acts directly on the laser internal temperature.

Description of related art

[0002] Semiconductor lasers are widely used nowadays in various state-of- the-art applications. As a few examples, quantum cascade lasers (QCL) constitute a very versatile source of coherent radiation in the mid-infrared spectral region that have shown numerous applications in precision molecular spectroscopy and trace gas monitoring, using either a distributed feedback structure or an external cavity to achieve single-mode emission. These lasers also have a potential for free-space optical communications in the 3-5 or 8-12 μιη atmospheric transmission windows. Distributed Bragg reflector (DBR) or distributed feedback (DFB) diode lasers in the near- and mid-infrared are other types of widely used light sources that also have numerous applications in spectroscopy (e.g. in trace gas sensing, high-resolution spectroscopy, optical thermometry, etc.), in optical metrology, sensing and optical

telecommunications (wavelength division multiplexing, coherent optical communications, etc.). Finally, vertical-cavity surface-emitting lasers (VCSEL) are semiconductor lasers with a very short vertical cavity that emit light from their surface with very nice beam properties, e.g. a circular beam with a low

divergence. They are used in particular in gas sensing and telecommunication applications. [0003] Most of the aforementioned applications, as well as many others, are quite demanding in terms of spectral purity of the laser source. A narrow- linewidth, highly coherent laser radiation is required in state-of-the-art applications and this cannot always be fulfilled by a solitary diode lasers or QCL. Even if semiconductor lasers have a quite narrow intrinsic linewidth in the kilohertz to tens of kilohertz range and even much below for QCLs (some hundred hertz) thanks to their close-to-zero linewidth enhancement factor, all semiconductor lasers suffer from some linewidth broadening processes that lead to a much larger linewidth observed in real experimental conditions. The observed linewidth is generally a few MHz for DFB diode lasers, in the range of several MHz for DFB-QCLs and several tens of MHz in VCSELs. The broadening mechanisms result from flicker noise (i.e. noise with a 1/f spectral dependence) that is generated in the laser itself due to the electrons transport through the multi-layer semiconductor structure that constitutes the laser gain region and cladding. This excess noise is internal to the laser structure and not related to the current driver used to control the laser. As an example, Figure 1 shows a comparison between the power spectral density (PSD) of the voltage noise measured on one hand across a 10-Ω resistor and on the other hand across a QCL with a similar differential resistance of 10 Ω. Whereas the white current noise of 1 nA/Hz^1/2 of the current source is observed with the resistor, a much higher 1/f-type noise is observed on the voltage measured across the QCL, which clearly shows the internal nature of this noise generation inside the laser structure.

[0004] It is known that the emission frequency of semiconductor lasers depends on the temperature of the device, as well as on the injection current that pumps the laser. For this reason, the use of a temperature control of the laser and of a stable and low-noise current source is a prerequisite to achieve a low frequency noise laser. Temperature-stabilization of a semiconductor laser is usually achieved by placing the laser sub-mount onto a thermo-electrical cooler (TEC) that heats or cools the laser and using a negative thermal coefficient (NTC) resistance as a temperature sensor in a regulation loop. But such a temperature regulation is quite slow (time constant in the range of seconds or longer), which results from the thermal inertia of the system and from the fact that the heating/cooling element as well as the temperature sensor are quite distant from the laser active region. Such a system is thus not able to detect and correct for fast temperature fluctuations of the laser active region, which is the relevant temperature that determines the laser emission frequency, but only regulates the average laser temperature. Furthermore, even with the use of a stable current source, any local temperature fluctuation within the laser structure will translate into fluctuations of the laser optical frequency. Such temperature fluctuations can be generated by the fluctuating electrical power dissipated in the structure resulting from the voltage noise that is induced by the electrons flux passing through the semiconductor structure as shown in Figure 1 in the case of a QCL The same type of noise also occurs in other types of

semiconductor lasers.

[0005] EP0618653, DE19839088, WO9219014 and GB2224374, disclose regulated semiconductor diode lasers in which the average internal temperature of the junction is stabilized, thereby reducing the long-term frequency drift of the laser. These realizations, however, do not provide a sizable narrowing of the linewidth.

[0006] A known method to characterize the frequency noise of a laser is to use an optical frequency discriminator to convert the frequency fluctuations of the laser into intensity fluctuations that can be detected using a photodiode, as described for instance by Tombez et al. [L. Tombez, S. Schilt, J. Di Francesco, T. Fuhrer, B. Rein, T. Walther, G. Di Domenico, D. Hofstetter, and P. Thomann, "Linewidth of a quantum cascade laser assessed from its frequency noise spectrum and impact of the current driver," Appl. Phys. B 109, 407-414 (2012)]. A possible implementation of such a measurement consists in using the side of a molecular transition in a reference gas cell as illustrated in Figure 2.

Alternatively, a resonance of a Fabry-Perot cavity or a long-arm unbalanced Mach-Zehnder interferometer can be used as well as a frequency discriminator.

[0007] Borri et al. suggested that frequency noise in QCLs is related to internal electrical fluctuations induced in the semiconductor structure by the electrons flux [S. Borri, S. Bartalini, P.C. Pastor, I. Galli, G. Giusfredi, D. Mazzotti, M. Yamanishi, and P. De Natale, "Frequency-noise dynamics of midinfrared quantum cascade lasers," IEEE J. Quantum Electron. 47, 984-988 (2011)]. This statement was verified experimentally by Tombez et al. who showed that frequency noise in a QCL is induced by internal electrical noise in the structure, such as fluctuations of the voltage across the laser when this later is driven by a low-noise current source [L. Tombez, S. Schilt, J. Di Francesco, P. Thomann, and D. Hofstetter, "Temperature dependence of the frequency noise in a mid-IR DFB quantum cascade laser from cryogenic to room," Opt. Express 20, 6851-6859 (2012)]. Alternatively, current fluctuations may be observed when supplying a laser with a constant voltage source.

[0008] As an example, a high degree of correlation between laser frequency fluctuations and laser voltage fluctuations has been observed when

simultaneously recording these two quantities in a QCL as illustrated in Figure 3. A similar behaviour also occurs for other types of semiconductor lasers such as DBR/DFB diode lasers or VCSELs. This was reported for instance by Richardson and Yamamoto in a laser diode [W. H. Richardson and Y. Yamamoto, "Quantum correlation between the junction-voltage fluctuation and the photon-number fluctuation in a semiconductor laser", Phys. Rev. Lett. 66,1963 (1991)] or by Goobar et al. in a 2-section DBR laser [E. Goobar, A. Karlsson, and S. Machida, "Measurements and theory of correlation between terminal electrical noise and optical noise in a two-section semiconductor laser," IEEE J. Quantum Electron. 29, 386-395 (1993)], even if Dandridge and Taylor stated on the contrary that junction voltage fluctuations are not correlated with the other types of noise , except when longitudinal mode hopping is occurring [A. Dandridge, and H.F. Taylor, "Correlation of Low-Frequency Intensity and Frequency Fluctuations in GaAIAs Lasers," IEEE J. Quantum Electron. QE-18, 1738-1750 (1982)]. [0009] A known method to realize a narrower linewidth semiconductor laser is to apply wavelength-selective feedback into a Fabry-Perot diode laser mounted in an external cavity configuration as discussed by Lang and Kobayashi in "External optical feedback effects on semiconductor injection laser

properties," IEEE J. Quantum Electron. QE-16, 347-355 (1980). The so-called external cavity diode lasers (ECDL) typically have a linewidth of 200-400 kHz and the improvement by factor 3-5 achieved in comparison with DFB lasers (with a typical linewidth of 1 MHz) is already a significant improvement for many applications. For this reason, ECDLs are often used in highly demanding applications in terms of laser spectral purity. However, the most common ECDL architectures (Littman-Metcalf and Littrow geometries) require an external mechanical moving part that makes the system larger and of increased complexity. [0010] Another established method to lower the frequency noise of a laser and to possibly reduce its linewidth is to use an active stabilization of the laser frequency to an optical reference, such as a molecular absorption transition, but the method requires a suitable molecular reference to which the laser frequency is compared. The method is most often applied to stabilize the average frequency of the laser to the molecular transition to compensate for the laser drift over long timescales (larger than one second).

[0011] The use of the voltage measured across the junction of a laser diode to derive an error signal for laser frequency stabilization was previously mentioned by E. Goobar, A. Karlsson, and S. Machida, in "Measurements and theory of correlation between terminal electrical noise and optical noise in a two-section semiconductor laser," IEEE J. Quantum Electron. 29, 386-395 (1993) and US-5,418,800 reports a method to control the optical phase of a laser from the junction voltage in a two-section semiconductor laser containing a gain part and a phase control part.

Brief summary of the invention

[0012] According to the invention, these aims are achieved by means of the object of the appended claims;

[0013] The inventive method enables achieving a reduction of the laser frequency noise and a narrowing of the associated linewidth, which is of similar magnitude as the improvement brought by the use of a wavelength-selective external cavity or even better, but by avoiding the use of such an external cavity which is critical in terms of alignment and long-term mechanical stability. The proposed method also avoids the use of any optical reference for the

stabilization and only makes use of an electrical measurement on the laser itself to generate an error signal for a high-bandwidth noise reduction feedback loop. The method originates from the observation of a high level of correlation between internal electrical fluctuations in the laser and frequency noise, as exemplified in Figures 3a-b for the particular case of a QCL, but which also applies to other types of semiconductor lasers. In contrast with prevalent art, the inventive method directly regulates the internal temperature of the laser gain medium over a large frequency band to achieve a narrow linewidth emission.

Brief Description of the Drawings

[0014] The invention will be better understood with the aid of the

description of an embodiment given by way of example and illustrated by the figures, in which:

Figure 1 plots Voltage noise measured across a 10-Ω resistor (plot 104) and across a QCL with a 10-Ω differential resistance (plot 106) when each device is driven by the same low-noise current source showing the importance of the 1/f noise induced internally in the QCL without any contribution from the current driver.

Figure 2 shows schematically an example of an optical set-up used to measure the frequency noise of a laser, using the side of a molecular transition in a gas reference cell as a frequency discriminator to convert frequency fluctuations (or frequency modulation - FM) of the laser into intensity fluctuations (or intensity modulation - IM) that are subsequently detected with a photodiode and analysed.

Figure 3a plots the correlation of the noise observed on the optical frequency of a QCL (upper frame, measured with a molecular frequency discriminator) and on the voltage across the laser (lower frame) in the time domain.

Figure 3b plots the frequency- and voltage-noise power cross-spectrum for the same arrangement as Figure 3a.

Figure 4 illustrates schematically the principle of the laser noise reduction method and shows a Constant-current laser driver l,a laser 2 , a PID servo controller 3 generating a feedback signal 4 regulating the laser temperature.

Figure 5 is an electrical scheme enabling to directly assess the fluctuations of the electrical power dissipated in a laser from the measurement of the fluctuations of the voltage and current in the laser. With a proper choice of the sense resistance R_S and of the differential gains Gi and G2 so that UR= UL, the fluctuations of the dissipated power δΡ are obtained from a direct sum of the measured voltage and current fluctuations δΙΛ and 5UR. Figures 6a-b illustrate the effect of the inventive method on the noise of a quantum cascade laser.

Figure 7 shows a typical frequency noise spectrum of a QCL and the frequency band that is relevant for the determination of the laser linewidth.

Detailed Description of possible embodiments of the Invention [0015] A schematic representation of the invention is shown in Figure 4. The method consists in detecting the electrical fluctuations across the

semiconductor laser 2 and using them as an error signal in a fast feedback loop to reduce the laser frequency noise in a large band and thus narrow its linewidth. In a first embodiment of the invention, the laser is driven by a low- noise constant current source 1. Despite the low-noise current characteristics of the current-source, voltage fluctuations due to the electronic transport in the laser structure are generated across the laser. These fluctuations (noise) are sensed. In a second embodiment of the invention, the laser is powered by a constant voltage source and the current fluctuations inside the laser are sensed using a probe resistance. In another embodiment of the invention, both the current and the voltage fluctuations in the laser are sensed in order to directly assess the fluctuations of the electrical power dissipated in the laser. The fluctuations of the electrical power can be obtained as a simple additive combination of the current and voltage fluctuations using a suitable

arrangement of sense resistances and signal amplification gains as schematized in Figure 5. In this case, it can be shown mathematically that if the gains Gi, G2, and the value of the shunt resistor R_S are chosen such that UR = Ui_ at the laser operating point, the fluctuations in the electrical power applied to the laser are proportional, to first-order approximation, to the fluctuations in (UR + UL). [0016] The detected electrical fluctuations are then amplified by a PID servo- controller 3 and fed back to an appropriate actuator 4 that is able to regulate the internal temperature of the laser (i.e. the temperature within the active region) at a high speed in order to reduce its fluctuations in a large band and narrow the resulting laser linewidth.

[0017] Various actuators can be used to control the laser internal

temperature. In a first embodiment of the invention, an integrated heating element such as an integrated heating-resistor, an integrated micro-Peltier element or any appropriate micro-heating element can be used. Such elements can be placed for instance on top of or lateral to the laser ridge in case of DFB diode lasers or DFB-QCLs, or at any other appropriate location. Preferably, the thermal actuator will be placed in intimate thermal contact with the laser junction, and will be miniaturized, in order to allow fast corrections of the temperature, as it will be explained later. [0018] In a different embodiment of the invention, heating of the laser can be achieved using an external light source such as a laser diode or an LED directed to the laser to be stabilized. Absorption of the external light beam in the laser to be stabilized results in a heating of the structure. Hence, controlling the optical power of the illuminating light source enables the internal temperature of the laser to be controlled with a high bandwidth. The external illumination can be realized over the entire laser surface at the same time, for instance on the laser top surface, but it can also be directed to the laser active region by the use of lateral exposure with a proper beam shape or it can be localized to some selected points on the top or lateral surface of the laser. [0019] The feedback loop that acts on the internal temperature of the laser compensates the effect of the electrical fluctuations. The result of the

stabilization is that both the electrical power fluctuations and the temperature fluctuations in the laser are significantly reduced in a wide frequency range, which in turn reduces the frequency instabilities of the laser and narrows its linewidth. Figure 6 shows the impact of the invention for the reduction of the frequency noise in a QCL. In the first half of the graphs, where the stabilization loop is inactive, both the voltage across the laser and the emission frequency show large fluctuations. The fluctuations of both parameters are significantly reduced, by one order of magnitude or more (see Figure 6(b)), when the stabilization loop is activated at time t = 1. In terms of laser linewidth, this improvement is comparable to the one brought by the use of an external cavity, which makes the proposed invention applicable to a wide range of applications where ECDLs are used. The invention can be applied in a similar manner to other types of semiconductor lasers.

[0020] More in detail, fast frequency fluctuations contribute to a large extend to the linewidth of a laser, i.e., fluctuations that occur at frequencies larger than a couple of kilohertz, typically larger than several tens of kilohertz and most often close to or higher than 100 kHz in typical semiconductor laser diodes and QCLs. As a result of the presence of 1/7 noise in semiconductor laser diodes and QCLs, the defined linewidth depends on the observation time, which should always be specified when stating the linewidth of a laser. An observation time in the 1-100 ms range is generally considered when talking about the linewidth of a semiconductor laser diode or of a QCL. The full width at half maximum (FWHM) linewidth can be directly obtained from the power spectral density (PSD) «¾„(/) of the laser frequency noise as:

where A corresponds to the surface of the frequency noise PSD for which -¾„(/) exceeds the ^-separation line defined as <¾„(/) = (81η(2)/ττ²) - /. In presence of 1/ / noise, this surface diverges at low Fourier frequency and a low cut-off frequency /₀ needs to be introduced in the calculation of the surface A and in the corresponding FWHM linewidth. The aforementioned observation time r₀ is directly related to the cut-off frequency by r₀ = l//₀. The cut-off frequency is thus 100 Hz for an observation time of 10 ms.

[0021] Figure 7 exemplifies the frequency noise PSD of a QCL. Frequencies up to about 100 kHz contribute here to the laser linewidth, the upper limit being given by the crossing point of the frequency noise PSD with the -separation line previously defined and shown as a dashed line in Figure 7. Therefore, reducing the linewidth with a stabilization loop needs a large feedback bandwidth that extends at least up to the crossing point of the frequency noise PSD with the -separation line, typically of 100 kHz or larger. The same is also valid for other semiconductor lasers, for example DFB laser diodes or VCSELs.

[0022] The inventors have realized that, in a large class of semiconductor lasers, the electrical parameters of the laser, for example the bias voltage, and its output wavelength, or equivalently its frequency, are highly correlated on an extended band of frequencies. Figure 3a shows, for example, that the frequency noise essentially copies the voltage noise across the laser and Figure 3b proves quantitatively that the high degree of correlation occurs at all visible

frequencies, up to more than 100 kHz. Thus the invention permits, by using a sufficiently fast feedback loop, the suppression not only of low frequency drifts, but also an effective reduction of the laser linewidth. In particular, the invention has been successfully applied to quantum cascade lasers with a significant reduction of the linewidth of such devices. It was not previously known that these devices allow frequency stabilization by control of their electrical parameters, as in the present invention.

[0023] Figure 6b shows that the stabilization method of the invention allows a reduction of the frequency noise in a band of frequencies up to 100 kHz. This implies that the feedback loop can correct temperature fluctuations occurring in the same frequency range.

[0024] Heating the laser active region by a controlled optical source is advantageous, because the heat delivered to the laser can be modified very quickly. The invention is not limited to this particular case, however, and could also utilise a miniaturized resistor or a thermal active element, in intimate thermal contact with the semiconductor laser active region, preferably realized on the same substrate as the laser, such that the internal temperature can be controlled in a band of frequencies sufficiently broad.

[0025] Preferably, the control loop has a bandwidth larger than 10 kHz, for example 100 kHz or, better, larger than 0.5 MHz or than 1 MHz.

Claims

1. Method of reducing the linewidth of a semiconductor laser and improving its spectral properties, which comprises: detecting an electrical fluctuation within the laser structure; deriving an error signal from the electrical fluctuation; controlling an internal temperature of the semiconductor laser based on said error signal.

2. Method according to claim 1, wherein the semiconductor laser is a quantum cascade laser.

3. Method according to claim 1, wherein the semiconductor laser is a DBR diode laser or a DFB diode laser or a VCSEL.

4. Method according to any of claims from 1 to 3, comprising measuring simultaneously a current and a voltage drop across the

semiconductor laser and assessing an electrical power dissipated in the semiconductor laser based on the measured current and voltage drop.

5. Method according to any of claims from 1 to 4, wherein the internal temperature of the semiconductor laser is controlled in a band of frequencies of at least 100 kHz.

6. Method according to any of claims from 1 to 4, wherein the semiconductor laser noise is described by a noise power spectral density, and its internal temperature is controlled in a band of frequencies extending at least up to the frequency at which the noise power spectral density crosses a line defined as S_Sv(f) = (81^η(2)/ττ²) - /.

7. Method according to any of claims from 1 to 6, wherein the error signal is applied to control the optical power of an illuminating light source directed to the semiconductor laser, thereby stabilising its temperature.

8. Method according to any of claims from 1 to 6, wherein the error signal is applied to control the power dissipated in a heating element such as a heating resistor, or to a thermally active element such as a Peltier cell, in thermal contact with the semiconductor laser, thereby stabilising its

temperature.

9. Method according to any of claims from 1 to 8, wherein the error signal is applied to a thermally active element such as a Peltier cell in thermal contact with the semiconductor laser, thereby stabilising its temperature.

10. Laser source including a semiconductor laser, a sense circuit for detecting an electrical fluctuation within the laser structure, a controller generating an error signal based on the electrical fluctuation detected in the sense circuit, and a thermal actuator, driven by the error amplifier such as to control the internal temperature of the semiconductor laser source, thereby narrowing its linewidth.

11. Laser source according to claim 10, wherein the semiconductor laser is a quantum cascade laser.

12. Laser source according to claim 10, wherein the semiconductor laser is a DBR diode laser or a DFB diode laser or a VCSEL.

13. Laser source according to any of claims from 10 to 12, wherein the internal temperature of the semiconductor laser is controlled in a band of frequencies of at least 100 kHz.

14. Laser source according to any of claims from 10 to 13, wherein the semiconductor laser noise is described by a noise power spectral density, and its internal temperature is controlled in a band of frequencies extending at least up to the frequency at which said noise power spectral density crosses a line defined as S_Sl/(f) = (81^η(2)/ττ²) · /.

15. Laser source according to any of claims from 10 to 14, wherein the sense circuit measures simultaneous a current and a voltage drop across the semiconductor laser, the controller being arranged to assess an electrical power dissipated in the semiconductor laser based on the measured current and voltage drop.

16. Laser source according to any of claims from 10 to 15, wherein the error signal drives a light source, directed to the semiconductor laser.

17. Laser source according to any of claims from 10 to 16, wherein the error signal drives a heating element or a thermoelectric element in thermal contact with the semiconductor laser.

18. Laser source according to the preceding claim, wherein the heating element or thermoelectric element is realized on the same substrate as the semiconductor laser.