Μm Laser Diode Research Articles

Three stage cascade diode lasers generating 500 mW near 3.2 μm

GaSb-based type-I quantum well 3.1–3.2 μm diode lasers with two- and three-cascade heterostructures were designed, fabricated, and characterized. Devices with ∼100-μm-wide aperture, 3-mm-long cavity, and anti-/high-reflection coated mirrors demonstrated continuous wave output power of 500 mW, threshold current density of ∼200 A/cm2, and peak power conversion efficiency of ∼7% at 17 °C. This corresponds to more than twofold improvement in terms of output power and efficiency as compared to standard diode lasers operating in the same spectral region. The experiment showed that the increase in the number of cascades from two to three led to critical enhancement of the differential gain and reduction of the threshold current density. Light p-doping of the AlGaAsSb graded section did not introduce extra optical loss but aided hole transport as required for realization of the efficient multi-stage cascade pumping scheme.

GaSb-based >3 µm laser diodes grown with up to 2.4% compressive strain in the quantum wells using strain compensation

We describe the growth of GaSb-based diode lasers with quinary AlInGaAsSb alloy waveguide/barriers and highly strained InGaAsSb type-I quantum wells, designed to emit at ~3.2µm at room temperature. To increase the compressive strain in the QWs above the pseudomorphic limit, we employ strain compensation in which the lower portion of the quinary alloy waveguide is slightly arsenic-rich as compared to the lattice-matched composition. Using this scheme, we fabricated a set of laser devices with incrementally increasing compressive strain in the quantum wells. With compressive strain reaching as high as ~2.4%, the device performance corroborates the notion that adding compressive strain in the quantum wells helps improve hole confinement and suppresses carrier leakage during operation near room temperature.

Research on the high indium content InGaAs multiple quantum wells wafers for λ > 1.55 μm laser diodes

The effects of TMIn flow rate and AsH3 flow rate on the photoluminescence spectra of the high indium content InGaAs multiple quantum wells for λ>1.55μm laser diodes have been investigated both experimentally and theoretically. The wavelength peak red-shifted about 4.8nm for increasing 1sccm H2 flow rate through TMIn under a AsH3 flow rate of 150sccm, while the wavelength shift increases to 6.5nm at a higher AsH3 flow rate of 300sccm. Results show that more AsH3 flow rate will promote much TMGa pyrolysis than TMIn in the high indium content InGaAs growth. Considering the influence of growth parameters, the longest wavelength of 1.889μm among the InGaAs/InGaAsP strained MQWs samples was obtained with high crystal quality.

Cascade type-I quantum well diode lasers emitting 960 mW near 3 μm

The cascade pumping scheme reduced the threshold current density of high power type-I quantum well GaSb-based λ ∼ 3 μm diode lasers down to ∼100 A/cm2 at room temperature. Laser heterostructures had single GaInAsSb quantum well gain stages connected in series by means of GaSb/AlSb/InAs tunnel junctions followed by InAs/AlSb electron injectors. Devices with densely stacked two and three gain stages and 100-μm-wide aperture demonstrated peak power conversion efficiency of 16% and continuous wave output power of 960 mW. Corresponding narrow ridge lasers demonstrated above 100 mW of output power. The experiment showed that the bandwidth of the gain and its rate of increase with current depended strongly on the thickness of AlSb layer separating electron injectors from quantum wells. The possible impact of electron injector interfaces and ionized impurities on the carrier scattering and recombination in the active quantum well is discussed.

High signal-to-noise ratio, single-frequency 2 μm Brillouin fiber laser.

A high optical signal-to-noise ratio (OSNR) single-frequency 2 μm Brillouin fiber laser (BFL) with watt-level output and high transfer efficiency is demonstrated for the first time to the best of our knowledge. The Brillouin pump is constructed with a two-stage thulium-doped fiber amplifier (TDFA) seeded by a 2 μm laser diode, providing 4.02 W average power with 1 MHz linewidth. Using an optimized length of 14 m for the Brillouin ring cavity, the BFL works stably in single-mode region with 8 kHz linewidth because of the linewidth narrowing effect. The transfer efficiency is 51% with 1.08 W output power and 62 dB OSNR for 3.22 W pump power.

120mJ Q-switched Er:YAG laser at 1645nm

A fibre-coupled 1.47µm laser diode end-pumped Er:YAG laser system comprising one oscillator and two single pass amplifiers is described. 120mJ pulses at a 30Hz repetition rate and 1.64µm emission wavelength are reported.

Dysprosium-doped PbGa_2S_4 laser generating at 43 μm directly pumped by 17 μm laser diode

In this Letter, we demonstrate the pulsed and CW operation of the Dy:PbGa(2)S(4) laser directly pumped by the 1.7 μm laser diode. In the pulsed regime (pulse duration 5 ms; repetition rate 20 Hz), the maximum mean output power of 9.5 mW was obtained with the slope efficiency of 9.3% with respect to the absorbed pump power. The generated wavelength was 4.32 μm, and the laser beam cross section was approximately Gaussian on both axes. Stable CW laser generation was also successfully obtained with the maximum output power of 67 mW and the slope efficiency of 8%. Depopulation of the lower laser level by 1.7 μm pump radiation absorption followed by 1.3 μm upconversion fluorescence was demonstrated. These results show the possibility of construction of the compact diode-pumped solid-state pulsed or CW laser generating at 4.3 μm in the power level of tens mW operating at room temperature.

Room temperature operated diffraction limited λ ≃ 3 µm diode lasers with 37 mW of continuous‐wave output power

Single spatial mode GaSb-based type-I quantum well diode lasers operating near 3 μm at room temperature were designed and fabricated by chlorine-free dry etching technique. The etching profile was optimised to minimise the optical loss. The 5.5 μm-wide ridge lasers demonstrated the same slope efficiency as that of 100 μm-wide multimode devices and generated 37 mW of continuous-wave output power at 17°C.

Robust interferometric frequency lock between cw lasers and optical frequency combs

A transfer interferometer is presented which establishes a versatile and robust optical frequency locking link between a tunable single frequency laser and an optical frequency comb. It enables agile and continuous tuning of the frequency difference between both lasers while fluctuations and drift effects of the transfer interferometer itself are widely eliminated via common mode rejection. Experimental results will be presented for a tunable extended-cavity 1.5μm laser diode locked to an Er-fiber based frequency comb.

Single spatial mode 2–2.2 µm diode lasers fabricated by selective wet etching

Diffraction limited λ = 2–2.2 µm ridge waveguide lasers with continuous wave output power near 100 mW at room temperature were fabricated with a two-step wet etching technique using the interface between the top cladding and waveguide core materials as an etch stopper. The devices did not suffer from lateral current spreading. The second optical mode was suppressed by the low lateral confinement. Experiments showed that reducing the ridge width below 6 µm did not affect the device efficiency or the output beam quality but led to an increase of the transparency current density. We speculate that this increase was associated with the mismatch between the wide modal size and the narrow current path in ridge lasers with suppressed lateral current spreading.

3 µm diode lasers grown on (Al)GaInSb compositionally graded metamorphic buffer layers

Diode lasers operating at 3 µm in continuous wave mode at room temperature were fabricated using metamorphic molecular beam epitaxy. The laser heterostructures have a lattice constant 1.3–1.6% bigger than that of the GaSb substrates. The mismatch between the epi-structure and the substrate lattice constants was accommodated by a network of misfit dislocations confined within linearly compositionally graded buffer layers. Two types of the buffers were tested—GaInSb and AlGaInSb. The laser heterostructures with Al-containing buffer layers demonstrated better surface morphology and produced devices with lower threshold and higher efficiency. At the same time the use of Al-containing buffers caused an excessive voltage drop across the laser heterostructure. Thus, a maximum continuous wave output power of 200 mW was obtained from lasers grown on GaInSb buffers, while only 170 mW was obtained from those grown on AlGaInSb buffers.

Supercontinuum generation from ~19 to 45 μmin ZBLAN fiber with high average power generation beyond 38 μm using a thulium-doped fiber amplifier

A mid-IR supercontinuum (SC) fiber laser based on a thulium-doped fiber amplifier (TDFA) is demonstrated. A continuous spectrum extending from ∼1.9 to 4.5 μm is generated with ∼0.7 W time-average power in wavelengths beyond 3.8 μm. The laser outputs a total average power of up to ∼2.6 W from ∼8.5 m length of ZrF4─BaF2─LaF3─AlF3─NaF (ZBLAN) fiber, with an optical conversion efficiency of ∼9% from the TDFA pump to the mid-IR SC. Optimal efficiency in generating wavelengths beyond 3.8 μm is achieved by reducing the losses in the TDFA stage and optimizing the ZBLAN fiber length. We demonstrate a novel (to our knowledge) approach of generating modulation instability-initiated SC starting from 1.55 μm by splitting the spectral shifting process into two steps. In the first step, amplified approximately nanosecond-long 1.55 μm laser diode pulses with ∼2.5 kW peak power generate a SC extending beyond 2.1 μm in ∼25 m length of standard single-mode fiber (SMF). The ∼2 μm wavelength components at the standard SMF output are amplified in a TDFA and coupled into ZBLAN fiber leading to mid-IR SC generation. Up to ∼270 nm SC long wavelength edge extension and ∼2.5× higher optical conversion efficiency to wavelengths beyond 3.8 μm are achieved by switching an Er:Yb-based power amplifier stage with a TDFA. The laser also demonstrates scalability in the average output power with respect to the pulse repetition rate and the amplifier pump power. Numerical simulations are performed by solving the generalized nonlinear Schrödinger equation, which show the long wavelength edge of the SC to be limited by the loss in ZBLAN.

Modular PbSrS/PbS mid-infrared vertical external cavity surface emitting laser on Si

A mid-infrared vertical external cavity surface emitting laser (VECSEL) based on undoped PbS is described herein. A 200 nm-thick PbS active layer embedded between PbSrS cladding layers forms a double heterostructure. The layers are grown on a lattice and thermal expansion mismatched Si-substrate. The substrate is placed onto a flat bottom Bragg mirror again grown on a Si substrate, and the VECSEL is completed with a curved top mirror. Pumping is done optically with a 1.55 μm laser diode. This leads to an extremely simple modular fabrication process. Lasing wavelengths range from 3–3.8 μm at 100–260 K heat sink temperature. The lowest threshold power is ∼210 mWp and highest output power is ∼250 mWp. The influence of the different recombination mechanism as well as free carrier absorption on the threshold power is modeled.

High gain 1.55 μm diode lasers based on InAs quantum dot like active regions

InP diode lasers with InAs quantum dot (QD) like active regions emitting at 1.55 μm have been fabricated. The QDs were grown in an As2 mode, which reduces the degree of elongation of the nanospecies yielding nearly circular shapes. Lasers with four to six dot layers show low absorption αi<10 cm−1 and high modal gain Γg0 of 10 cm−1 per QD layer (QDL) and above. The high gain values are compatible with an inhomogeneous linewidth that is much narrower than in quantum dash material, which is the common nanoscale gain material in the InP system.

Optically-pumped mid-IR phosphor using Tm^3+-sensitized Pr^3+-doped KPb_2Cl_5

Efficient energy transfer at room temperature from Tm3+ to Pr3+ has been demonstrated in co-doped KPb2Cl5. Because of the low phonon energies in KPb2Cl5, the energy transfer processes result in the conversion of 805 nm laser diode pump energy to a broad band of mid-IR radiation between 3500to5500 nm. Energy transfer pathways, rates, and quantum efficiencies are evaluated. Results show that the material is suitable as a phosphor for the 4to5⁢ μm spectral range that can be optically pumped with low-cost 0.8⁢ μm laser diodes.

A high-peak-power UV picosecond-pulse light source based on a gain-switched 155 μm laser diode and its application to time-resolved spectroscopy of blue-violet materials

We generated sub-kilowatt peak-power and 6-ps duration 390-nm optical pulses via the fourth harmonic generation of amplified optical output from a gain-switched 1.55-microm laser diode. We obtained a power-conversion-efficiency of 12% from 1.55-microm to 390-nm light, and subsequently applied the ultraviolet pulses to time-resolved spectroscopy of blue-violet luminescent materials, including a Coumarine dye solution and nitride semiconductor materials using single-photon and two-photon excitation schemes.

Integrated wavelength monitoring in a photonic-crystal tunable laser diode

A photonic-crystal tunable 1.55 μm laser diode is fitted with a wavelength monitor on its rear side. The 250-μm long laser based on a coupled-cavity design has approximately 15 nm tunability. The wavelength monitor collects and differentially feeds two-photodetecting areas, thanks to a mode conversion to a higher-order mode (a mini-stopband), followed by tunneling through a thin clad. The layout is numerically optimized to minimize unwanted reflections. Electrical cross-talk was prevented through guard rings and trenches. The correlation between wavelength and the monitor photocurrent ratio demonstrates a 10–20 GHz stabilization capability, or a 15 nm monitoring range.

Detuning characteristics of the linewidth enhancement factor of a midinfrared quantum cascade laser

The linewidth enhancement factor of a 5μm distributed feedback (DFB) quantum cascade laser above the threshold current detuned from the gain peak was investigated. This laser oscillates in two DFB modes located on either side of the gain peak. Using the optical feedback self-mixing method, we have observed variations in the linewidth enhancement factor, deviating from zero. The signs of the two DFB modes are both negative, which was verified by comparing their waveforms with that of conventional 1.55μm laser diodes, suggesting that the gain curve is asymmetric with respect to the gain peak.

Frequency stabilization of a 13 μm laser diode using double resonance optical pumping in the 5P_3/2-6S_1/2 transition of Rb atoms

The frequency stabilization of a laser diode in the 1.3 microm region using double resonance optical pumping (DROP) spectrum in the 5P(3/2)-6S(1/2) transition of (87)Rb atoms is demonstrated. The signal-to-noise ratio of the DROP spectrum is approximately ten times higher than that of the previous optical-optical double resonance spectrum. The spectral linewidth of the DROP measures 8.4 MHz. When the frequency of a 1.367 microm laser diode is stabilized to the DROP spectrum, the frequency stability is 9 x 10(-12) after 100 s. Also, the wavelength of the frequency-stabilized laser locked to the 5P(3/2)-6S(1/2) transition using a wavelength meter is measured.

PbSnTe double-hetero junction laser diode and its application to mid-infrared spectroscopic imaging

Abstract In-doped n-PbTe/p-PbSnTe/Tl-doped p-PbTe LPE layers were successively grown for the fabrication of mid-infrared (λ∼7.5 μm) laser diodes (LDs) by using the constant temperature liquid phase epitaxy on (1 0 0) oriented p-type PbTe substrates. Substrate materials were prepared by the stoichiometry-controlled Bridgman method. Whereas, the present double hetero LDs have a simple broad contact structure, the threshold current density for lasing is as low as ∼200 A cm 2 . The emitted wavelength is tunable by changing the operation temperature. PbSnTe DH-LD was applied for the mid-infrared spectrum measurements of glycine and the spatial distribution imaging was obtained by using the specific absorption line (fingerprints), which vibration mode is originated from the skeleton oscillation of molecules. Another demonstration for the application of PbSnTe DH-LD spectrometer is the imaging of carrier distribution on Si wafers based on the change of free carrier absorption coefficients.