This PDF file contains the front matter associated with SPIE Proceedings Volume 9002, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

We describe the development of hybrid quantum well (QW)/quantum dot (QD) active elements to achieve broad spectral bandwidth spontaneous emission and gain. We have previously reported that the placement of the QW within the active element is a critical factor in obtaining broad spectral bandwidth emission. We now present new designs to further broaden the spontaneous emission from hybrid structures by increasing the number of QD layers and dot density, and by using QDs with wider state-separation. Introducing chirped QD layers reduced the modulation in the spontaneous emission spectra, and by utilising self-heating effects and state-filling, a spontaneous emission with 3dB line-width of 350nm is obtained.

Raman lines are often obscured by background light or fluorescence especially when investigating biological samples or samples containing impurities. Shifted excitation Raman difference spectroscopy (SERDS) is a technique to overcome this. By exciting the sample with two slightly shifted wavelengths, it is possible to separate the Raman lines and distortions. In this paper, monolithic dual wavelength DBR diode lasers meeting the demands of Raman spectroscopy and SERDS will be presented. The wavelengths are stabilized and selected by using deeply-etched 10th order surface gratings with different periods manufactured using i-line wafer stepper lithography. Two possible resonator concepts, i.e. a mini-array of two parallel DBR RW-lasers and a Y-branch DBR laser, will be compared. Established excitation wavelengths for Raman spectroscopy at 671 nm and 785 nm are chosen. The total laser length is 3 mm; the ridge width is 2.2 μm for the 785 nm devices and 5 μm for the 671 nm lasers. The length of the DBR gratings is 500 μm. The devices at 671 nm reach output powers up to 100 mW having an emission width smaller than 12 pm (FWHM). The 785 nm lasers show output powers up to 200 mW and a narrow emission below 22 pm. For the dual wavelength lasers the spectral distance between the two excitation lines is about 0.5 nm as targeted. The power consumption at both wavelengths is below 1 W. These data proof that the devices are well suited for their application in portable Raman measurement systems such as handheld devices using SERDS.

Conventional singlemode semiconductor DFB and VCSEL lasers used in high resolution spectroscopy are often required to operate at specific, custom wavelengths, such as those associated with gas absorption lines. We present the results of work to develop alternative sources in the 1550nm and 1650nm regions, the latter coinciding with an absorption line of methane. Custom wavelength Bragg gratings have been used to stabilize the output of external cavity lasers implemented in both optical fiber and planar silica-on-silicon integrated circuits, using commercially available semiconductor gain chips, to give laser output at 1648 and 1649 nm, respectively. Thermal expansion or mechanical strain of the Bragg grating offers a suitable wavelength tuning mechanism. Results are presented including the wavelength tuning range, output power, relative intensity noise (RIN), side-mode suppression and linewidth of devices for application in high resolution gas spectroscopy. The different methods of writing Bragg gratings in optical fiber and planar silica-on-silicon allow a high degree of flexibility in the choice of emission wavelength.

Single-mode lasers in the spectral region between 620 nm and 630 nm are still realized using complex laser systems, such as ring-dye laser or using non-linear frequency shifted lasers, when used in applications such as laser cooling of beryllium ions or spectroscopy on rare earth elements. Direct emitting AlGaInP based diode lasers offer a much simpler approach to this wavelength range, but so far lack a suitable beam quality and spectral purity. Recently distributed Bragg reflector (DBR) ridge waveguide lasers (RWL) were developed for the 630 nm to 640 nm region. Building on this knowledge CAMFR simulations were performed to find suitable grating periods and duty cycles to obtain emission wavelengths below 630 nm. The grating itself was then introduced by stepper lithography and reactive ion etching into the laser structure. The manufactured DBR-RWLs show laser emission at 628.5 nm and 626.5 nm at a temperature of 15°C with threshold currents below 150 mA. The spectral emission shows single-mode operation with side mode suppression ratios > 20 dB. Two DBR-RWLs with the shorter wavelength were packaged into sealed TO-3 housings to provide a small-sized non-condensing environment with temperatures down to -25°C. When cooled internally to about 0°C, an emitted power of more than 50 mW was measured at a wavelength of 626.0 nm. At this operation point a diffraction-limited single longitudinal mode was observed that allowed a heterodyne measurement where a spectral width below 1 MHz was obtained. These new diode lasers have the potential to drastically miniaturize existing set-ups for quantum information processing.

We present the first demonstration of InAs/InP Quantum Dash based single-section frequency comb generator designed for use in photonic integrated circuits. The laser cavity is closed using a specific Bragg reflector without compromising the mode-locking performance of the laser. This enables the integration of single-section mode- locked laser on photonic integrated circuits as on-chip frequency comb source. As a demonstration, we integrate the Fabry Perot laser with a semiconductor optical amplifier. Such a device could be used for amplification or modulation of the frequency generated comb. We thus investigate the device operation to obtain a NRZ modulated comb.

In this work we present a laser cavity with a spatial light modulator (SLM), which allows for arbitrary phase and amplitude manipulation. In comparison to previous setups, it allows the manipulation of spectral components inside the laser cavity without the introduction of spatial chirp. An electrically driven ultrafast semiconductor laser system is used for proper alignment of the laser cavity. We were able to demonstrate that the gain of the laser supports mode-locking operation over a spectral range greater than 12 nm with a central wavelength of 850 nm. This bandwidth has the potential to generate sub 150 fs pulses.

290 fs optical pulses have been demonstrated using a two-section, passively mode-locked InAs quantum dot laser with a proton bombarded absorber, at a repetition rate of 20 GHz, centred around 1220 nm. Optical pulsewidth measurements between 300 K and 20 K, reveal that the pulsewidth decreases by a factor of 29, from 8.4 ps at 250 K to 290 fs at 20 K with a corresponding change in optical bandwidth. Rate equation analysis shows that this is due to the decreased emission rate compared to the recombination rate at 20 K in the random population regime. Random population ensures the dots act as independent oscillators across the entire inhomogeneous distribution thereby allowing access to the full gain spectrum. Proton bombardment of the absorber section shortens the recovery time of the absorber while the dots remain randomly populated.

Detailed experimental investigations of the generation of high-energy short infrared and green pulses with a mode-locked multi-section distributed Bragg reflector (DBR) laser in dependence on the lengths of the gain section and the saturableabsorber (SA) section as well the corresponding input currents and reverse voltages, respectively, are presented. The laser under investigation is 3.5 mm long and has a 500 μm long DBR section. The remaining cavity was divided into four 50 μm, four 100 μm, two 200 μm and eight 250 μm long electrically separated segments so that the lengths of the gain and SA sections can be simply varied by bonding. Thus, the dependence of the mode-locking behavior on the lengths of the gain and SA sections can be investigated on the same device. Optimal mode-locking was obtained for absorber lengths between L_Abs = 200 μm and 300 μm and absorber voltages between U_Abs= -2 V and -3 V. A pulse length of τ ≈ 10 ps, a repetition frequency of 13 GHz and a RF line width of less than 100 kHz were measured. An infrared peak pulse power of 900 mW was reached. The FWHM of the optical spectrum was about 150 pm. With an 11.5 mm long periodically poled MgO doped LiNbO₃ crystal having a ridge geometry of 5 μm width and 4 μm height green light pulses were generated. With an infrared pump peak power of 900 mW a green pulse energy of 3.15 pJ was reached. The opto-optical conversion efficiency was about 31%.

A 245.3 nm deep ultraviolet optically pumped AlGaN based multiple-quantum-well laser operating at room temperature is described. Epitaxial growth was performed by metalorganic chemical vapor deposition on a c-plane bulk AlN substrate at a growth temperature of ~ 1130 °C. The wafer was fabricated into cleaved bars with a cavity length of ~1.45 mm and the lasing threshold was determined to be 297 kW/cm² under pulsed 193 nm ArF excimer laser excitation. A further ~20% reduction in threshold pumping power density was observed with six pairs of SiO₂/HfO₂ distributed Bragg reflector deposited at the rear side of facets.

We investigate the influence of the epitaxial layer roughness on the far-field profile of the optical mode in gallium Nitride-based, c-plane ridge waveguide laser diodes. Occasionally, we observe long-range growth instabilities leading to a periodical modulation of the surface. Amplitude and period of this surface roughness is typically on the order of a few 10nm and 20 μm, respectively. Using different characterization techniques, we investigate the influence of the surface roughness on the vertical mode profile along the fast axis in the far-field, in particular the contribution of light scattering at the rough waveguide interfaces, as well as that of substrate modes.

We demonstrate pulse periods from 0:13 to 10 ns of GaN{based ridge waveguide laser diodes with monolithically integrated absorbers in the regimes of relaxation oscillations and self{Q{switching as function of gain current and absorber voltage. We introduce a simple model for the self{Q{switching regime, describing the pulse period in terms of current injection and spontaneous emission (including Auger recombination), only. At reverse voltages larger than 35V the modal absorption exceeds 500 cm^-1, which cannot be explained solely by transitions of bound states in the quantum wells. Calculations based on wavefunction overlap and quantum con ned Stark e ect (QCSE) predict a decrease of absorption at such large bias. In contrast, we show experimental ndings, proving that the absorption further increases. Due to the strong tilt of the band pro le in this regime, we take into account the Franz{Keldysh e ect in the barriers and the waveguide and discuss its possible in uence on the absorption, leading to an increased absorption at large reverse bias.

Aurrion’s heterogeneous integration process enables high performance active components such as lasers, modulators, and photodetectors to be elegantly integrated on a silicon photonics platform with high performance passive components. This platform also offers the unique capability to combine different types of active devices with separately optimized materials on the same wafer, die, and photonic integrated circuit. Similarly, devices and photonic integrated circuits operating in different wavelength bands can be formed within the same wafer and die. Experimental demonstrations show that these active components can achieve performance on par with commercially available discrete III-V components. In this paper we will discuss the advantages of Aurrion’s heterogeneous integration platform and discuss prototype demonstrations.

Silicon photonics is attracting large attention due to the promise of fabricating low-cost, compact circuits that integrate photonic and microelectronic elements. It can address a wide range of applications from short distance data communication to long haul optical transmission. Today, practical Si-based light sources are still missing, despite the recent demonstration of an optically pumped germanium laser. This situation has driven research to the heterogeneous integration of III-V semiconductors on silicon through wafer bonding techniques. This paper reports on recent advances on integrated hybrid InP/SOI lasers and transmitters using a wafer bonding technique made in particular at III-V Lab, France.

Photonic crystal surface emitting lasers (PCSELs) have recently been achieved with both a single spectrum and narrow spot beam pattern under several hundred mW of output power. Even though the high coherence properties of PCSELs are expected to be used for various applications, we have focused on a pumping light source for a wavelength conversion system in this work. We fabricated a 1.06 μm PCSEL with a square lattice 2D photonic crystal in which the lattice period corresponded to the lasing wavelength to obtain green light. The fabricated device had a narrow spot beam pattern of less than 0.5 degrees and a single spectrum at 1068 nm under CW output power of more than 200 mW despite the broad emitting area of 200 × 200 μm2. The wavelength conversion system used single pass second-harmonic generation (SHG) that consisted of only the PCSEL and 50 mm long bulk MgO doped periodically with poled lithium niobate (MgO:PPLN) as a nonlinear medium, i.e., it was a lens-free system. It was important to maintain the high brightness of the pumping light in this system with a single spectrum through the MgO:PPLN. As a result, SHG light was obtained at 534 nm with a narrow spot beam pattern, which followed the beam quality of the PCSEL under CW operation.

Cascade GaSb-based type-I quantum well diode lasers were designed and fabricated. Cascade pumping was achieved utilizing efficient interband tunneling through "leaky" window in band alignment at GaSb/InAs heterointerface. The 100% efficient carrier recycling between stages was confirmed by twofold increase lightcurrent characteristics slope of two-stage 2.4 – 3.3 μm cascade lasers as compared to reference single-stage devices. Moderate internal optical loss increase was observed in cascade lasers with interband injector located near the optical mode peak. Cascade pumping scheme increased the continuous wave output power of room temperature operated 2.4 - 3 μm semiconductor lasers and led to improved power conversion efficiency.

Mid-infrared spectral region (2-4 μm) is gaining significant attention recently due to the presence of numerous enabling applications in the field of gas sensing, medical, environmental and defense applications. Major requirement for these applications is the availability of laser sources in this spectral window. Type-I GaSb-based laser diodes are ideal candidates for these applications being compact, electrically pumped, power efficient and able to operate at room temperature in continuous-wave. Moreover, due to the nature of type-I transition; these devices have a characteristic low operation voltage, typically below 1 V, resulting in low power consumption, and high-temperature of operation. In this work, we present recent progress of 2.7 μm – 3.0 μm wavelength single-spatial mode GaSb type-I laser diode development at Brolis Semiconductors. Experimental device structures were grown by solid-source multi-wafer MBE, consisting of an active region with 2 compressively strained (~1.3 %-1.5 %) GaInAsSb quantum wells with GaSb barriers for 2.7 μm devices and quinternary AlGaInAsSb barriers for 3.0 μm devices. Epi-wafers were processed into a narrow-ridge (2-4 μm) devices and mounted p-side up on CuW heatsink. Devices exhibited very low CW threshold powers of < 100 mW, and single spatial mode (TE00) operation with room-temperature output powers up to 40 mW in CW mode. Operating voltage was as low as 1.2 V at 1.2 A. As-cleaved devices worked CW up to 50 deg C.

We report GaSb-based broad-area (BA) lasers, ridge-waveguide (RWG) lasers and laterally coupled distributedfeedback (LC-DFB) lasers grown metamorphically on GaAs substrates. Despite the large lattice mismatch, a high crystal quality laser structure was achieved. All lasers operate continuous-wave at room temperature with emission wavelengths near 2-μm. BA lasers emit total output power up to 70 mW at 15°C with low internal loss and low thermal resistance compared to those of pseudomorphic lasers. Single lateral mode RWG lasers demonstrate stable output power over a 1000-hour test and operate continuous-wave up to 70°C. LC-DFB lasers emit total output power of up to 49 mW in a single longitudinal mode and side-mode suppression ratio of 25 dB at a heat-sink temperature of 15°C.

Active region designs of QCLs containing composite barriers based on AlAs allow short wavelength emission, improved injection efficiency, and high values of T₀ and T₁. On the other hand, AlAs introduces challenges, not only in strain compensation and growth, but also in effects on thermal management, thermal stability, and scattering. Leakage current, allowing electrons to bypass transitions between upper and lower laser levels occur due to scattering of electrons into higher-lying states via phonons and interface roughness scattering. This interface roughness scattering is exacerbated by large values of ΔE_c and by the rms roughness itself, both of which are pronounced at the AlAs/InGaAs interface. The resulting leakage current noticeably reduces the slope efficiency, leading to more heating to achieve a given emission power. Efficient thermal management requires a buried heterostructure design; the re-growth of InP:Fe, however, needs to be carried out at temperatures consistent with maintaining the highly strained AlAs/InGaAs interfaces. This paper describes the physics of intersubband electron scattering due to strained interfaces and some partially optimized structures with J_th = 1.7 kA/cm² at 300 K, slope efficiency η = 1.4 W/A, T₀ = 175 K, and T₁ = 550 K. Re-growth of InP:Fe using gas-source MBE at substrate temperatures below 550°C results in packaged lasers with 7 μm width having high thermal conductance.

Interband cascade lasers are mid-infrared semiconductor lasers that are very promising for low power consumption applications in the 3-6 μm spectral range. Therefore, they are ideal for gas sensing of hydrocarbons. Combining interband transitions utilized in diode lasers with a cascading scheme widely exploited in intersubband transition based quantum cascade lasers, interband cacscade lasers can operate at threshold current densities around or below 100 Acm-2 at room temperature. Distributed feedback interband cascade lasers emit at room temperature continuous wave output powers in the mW range and above, and their side mode suppression ratio can well exceed 20 dB.

We report a narrow-ridge interband cascade laser emitting at λ ≈ 3.5 μm that produces up to 592 mW of cw power with a wallplug efficiency of 10.1% and beam quality factor of M2 = 3.7 at T = 25 °C. Furthermore, devices from a large number of wafers with similar 7-stage designs and wavelengths spanning 2.8-4.7 μm exhibit consistently higher pulsed external differential quantum efficiencies than earlier state-of-the-art ICLs.

The III-nitride semiconductors have been proposed as candidate materials for new quantum cascade lasers in the nearinfrared (1.5-3 μm), and far-infrared (30-60 μm), due to the large conduction-band offset between GaN and Alcontaining alloys (>1 eV), and the large longitudinal optical (LO) phonon energy (90 meV), respectively. The challenges of III-nitride intersubband devices are twofold: material and design related. Due to large electron effective mass, the nitride intersubband materials require the ability to fine-tune the atomic structure at an unprecedented sub-nanometer level. Moreover, the III-N materials exhibit built-in polarization fields that complicate the design of intersubband lasers. This paper presents recent results on c-plane nitride resonant-tunneling diodes that are important for the prospects of farinfrared nitride lasers. We also report near-infrared absorption and photocurrent measurements in nonpolar (m-plane) AlGaN/GaN superlattices.

InP-based antimony-free In_0.53Ga_0.47As/InAs/In_0.53Ga_0.47As strained triangular quantum well lasers have been demonstrated for the light sources with wavelength beyond 2 μm. Theoretical estimation shows that the triangular quantum well owns the longer emission wavelength than the rectangular quantum well with the same strain extent. The triangular quantum well was formed experimentally by using gas source molecular beam epitaxy grown digital alloy, and the growth temperature of the triangular quantum wells was optimized. The triangular quantum well lasers with emission beyond 2.2 μm under continuous-wave operation at temperatures higher than 330 K have been demonstrated. The performances of the triangular quantum well lasers are improved comparing to those of InAs rectangular quantum lasers with the nearly same lasing wavelength.

Long-wavelength diode lasers, emitting at 1.5x μm, have been optimized for maximum continuous-wave (CW) electroto- optical power conversion efficiency (PCE) and output power. A maximum CW PCE value of 50% is achieved at room-temperature from a 0.10 x 1.5 mm² diode laser with a CW output power of 2.5 W from a laser structure with a single-quantum-well (SQW) active region. Reliability tests show no degradation when run at 5A, 40°C for < 4000 hours of operation.

Due to the narrow absorption peak of Yb in glass at 975nm, a wavelength stabilisation is necessary for pump laser diodes. Our original approach is to use a Distributed Feedback broad area structure which is made possible since we use an Aluminium free active region laser structure. The target is to obtain high optical power of 10W with both a reduce wavelength evolution with temperature and a low spectral width. On a 3mm x 100 μm BA Fabry Perot laser, we have obtained a high power of 10W. Design, fabrication and results on BA DFB lasers will be presented, showing a low spectral width at 976nm at 3W output power.

Catastrophic optical damage (COD) is one of the limiting factors preventing diode lasers from reaching even higher optical output powers. We apply different techniques to AlGaAs/GaAs based quantum well diode lasers emitting at 808 nm in order to investigate the temperature kinetics during COD. The latter was subject to controversial discussions in the past. We experimentally verify the presence of temperatures as high as ≈1600°C at the defect front during the entire defect growth process. Locations passed by this front are affected by a rapid heating-cooling-cycle. Our results allow a deeper understanding of the mechanisms related to COD on the microscopic and macroscopic scale.

GaAs-based high power diode lasers are the most efficient source of optical energy, and are in wide use in industrial applications, either directly or as pump sources for other laser media. Increased output power per laser is required to enable new applications (increased optical power density) and to reduce cost (more output per component leads to lower cost in $/W). For example, laser bars in the 9xx nm wavelength range with the very highest power and efficiency are needed as pump sources for many high-energy-class solid-state laser systems. We here present latest performance progress using a novel design approach that leverages operation at temperatures below 0°C for increases in bar power and efficiency. We show experimentally that operation at -55°C increases conversion efficiency and suppresses thermal rollover, enabling peak quasi-continuous wave bar powers of P_out > 1.6 kW to be achieved (1.2 ms, 10 Hz), limited by the available current. The conversion efficiency at 1.6 kW is 53%. Following on from this demonstration work, the key open challenge is to develop designs that deliver higher efficiencies, targeting > 80% at 1.6 kW. We present an analysis of the limiting factors and show that low electrical resistance is crucial, meaning that long resonators and high fill factor are needed. We review also progress in epitaxial design developments that leverage low temperatures to enable both low resistance and high optical performance. Latest results will be presented, summarizing the impact on bar performance and options for further improvements to efficiency will also be reviewed.

The photonic-crystal surface-emitting laser (PCSEL) is an attractive semiconductor laser in which a thin two-dimensional photonic-crystal (2D-PC) layer is incorporated into the ordinary broad area edge-emitting laser structure to control the longitudinal-transverse mode owing to diffraction. In principle, the zero group velocity effect at the band edge of the 2D-PC is utilized as a resonator and can be used for the unique properties including large-area coherent oscillation as well as arbitrary beam controlling, which includes the polarization, beam patterns, directions, and generation of vector beams. We investigated the PCSEL toward realizing a practical device that has high power and high beam quality. Here, we show our recent progress. The device structure, which consists of an InGaAs/AlGaAs material system on n-GaAs substrates, is based on an ordinary broad area edge-emitting laser structure except it has a thin 2D-PC layer. The 2D-PC layer is placed near the active layer, and both are embedded between the p and n cladding layers. It is fabricated by using EB lithography, dry etching, and regrowth or MOCVD. The square emitting area has side of 200 micrometers, and transverse modes are well controlled in the entire region. The output power is more than 0.75 W with a single wavelength of 966 nm, and the narrow beam divergence is as narrow as 1° under continuous wave (CW) operation at room temperature. The beam quality is superior with an M² of 1.1, which is almost the same as that of the ideal Gaussian beam.

An integrated widely wavelength tunable mid-IR source is demonstrated. This three-section quantum cascade laser consists of a Fabry-Perot section placed between two super-structure grating (SSG) sections. The emission wavelength of the SSG-DBR QCL can be well controlled by varying the injection currents to the two grating sections of the device. The wavelength can be tuned from 4.58 μm to 4.77 μm (90 cm^-1) with a supermode spacing of 30 nm. The SSG-DBR QCL can be a compact replacement for the external cavity QCL. It has great potential to achieve much further tuning ranges for many mid-IR sensing applications.

In this work we introduce the design, optimization, simulation and experimental characterization results of a 30-to-1 wavelength multiplexer for a Distributed Feedback Quantum Cascade Laser (DFB QCL) laser array operating in the 7- 8.5 μm (mid-long) infrared (IR) range based on an Echelle mirror using a dual Rowland circle grating scheme. This design is proposed in order to achieve a continuous tuning range overcoming the limited tunability of individual QCLs. The design is based on a DFB-QCL array with wavelength spacing of 0.05 μm, aiming to reducing coupling between the slab waveguides to a minimum. We discuss the design parameters such as the order of diffraction, the operation wavelength range in the slab waveguides and the position of both the input and output waveguides are optimized for obtaining higher output power in the overall wavelength range of the multiplexer device than in a single Rowland circle grating scheme, providing an improvement in channel transmission. Other design characteristics, such as the structure scalability and reduction in size for these devices are considered and studied, including the input/output waveguide optimization as a function of parameters such as waveguide width, etching depth and wavelength. A systematic process is presented for all steps in the design of these devices, comparing both simulated and experimental results, placing them as suitable options when compared to other IR multiplexer schemes in terms of size and transmission.

We have recently described a method to analyze the leakage current (J_leak) in quantum-cascade lasers (QCLs) for carriers scattering into higher minibands due to LO-phonon absorption. In his presentation we analyze J_leak due to elastic scattering. We illustrate how at low temperature, when inelastic scattering is negligible, this current becomes significant for devices operating at high electron temperatures. Measuring J_leak above threshold we are able to investigate the effect of electron temperature on the differential quantum efficiency. This procedure is supported by a self-consistent calculation of the rate equations based on a phenomenological scattering-rate model. We apply our approach and measure J_leak above threshold as a function of electron temperature for a QCL emitting near 5.4 μm operated at a low duty cycle and a heat sink temperature of 80 K. This current is then modeled using a thermally activated, electron temperature-driven, scattering model based on intersubband interface roughness scattering. As a result, a reduction in the upper laser state population by ∼35% is estimated due to the effect of increased electron temperature. A decrease of the quantum efficiency of ∼80% is estimated for an electron temperature of 400 K.

Based on the nature of ultra-fast carrier life time in semiconductor quantum well, optical modulation of quantum cascade laser offers an unique way to control intersubband transition through interband transition. This method circumvents the problem of parasitic effects associated with electrical modulation, resulting in a high modulation bandwidth. In addition it allows for fast wavelength modulation on standard type quantum cascade lasers by directly injecting charge carriers to laser active region with near-infrared optical excitation. Here, we demonstrate the first infrared spectroscopic measurement conducted with this all-optical modulation approach. Using wavelength modulation spectroscopy, a 1st order derivative spectrum of methanol vapor gas is observed. Optically based wavelength modulation up to 200 MHz is purely induced by pumping the front facet of quantum cascade laser with an intensity-modulated 1550 nm DFB laser. Compared with conventional direct absorption approach, the noise equivalent sensitivity is improved by a factor of 10 by adding optical modulation in a non-optimized system.