2.1 Gallium antimonide (GaSb) laser diodes

The III-V compound semiconductor material system (AlGaIn)(AsSb) constitutes an alternative basis for the realisation of diode lasers in the mid-IR. GaInAsSb is either latticed matched to GaSb substrates or strained and is used as the active layer with a direct bandgap transition covering the λ ∼ 1.8–3.4 μm wavelength region. For the barrier and cladding layers, AlGaAsSb is well suited because of its larger bandgap energy and lower refractive index compared to GaInAsSb. Molecular-beam epitaxy (MBE) is the method of choice and most widely used in the wafer growth. The laser structures are grown on (100)-orientated n-doped GaSb substrates but due to the lower demand for GaSb substrates compared to InP the growth cost is higher and substrate quality lower. Conventional fabrication of distributed-feedback (DFB) lasers incorporating buried gratings for longitudinal mode selection is challenging in the GaSb material system due to the difficulty of epitaxial regrowth with the high Al concentrations in the cladding layers. Separately, processing options are quite limited for GaSb due to the cladding material being hygroscopic.

2.1.1 Laterally coupled distributed-feedback

A proposed method for DFB fabrication which was recently demonstrated by the Jet Propulsion Laboratory (JPL) [18], makes use of Bragg gratings etched alongside a ridge waveguide to form a laterally coupled distributed-feedback (LC-DFB) as shown in Figure 2This approach enables fabrication of single-longitudinal-mode laser following a single epitaxial growth process.

Figure 2.

(a) Cross-section scanning-electron micrograph of a 3-μm-wide laser ridge topped with a titanium-platinum-gold contact layer. (b) Top view of the LC-DFB laser structure, with a cross section of the grating (inset) [18].

Figure 4.

Calculated bandgap wavelength for InGaAs on InP as a function of In composition. The upper horizontal axis shows the mismatch strain of InGaAs with respect to InP. Red dashed line indicates the lattice constant of InP 5.869A [13].

2.2.1 InP-DFB

InP-based DFB lasers have been extensively developed with wavelengths at 1.3 and 1.55 μm for fibre optic communications over the last three decades. Similar processing techniques can be used for fabrication of single mode lasers operating in the 2 μm wavelength range. NTT-Japan demonstrated a DFB laser with an emission wavelength of 2.051 μm and output power of 10 mW [22]. A schematic of their buried heterostructure DFB grating is shown in Figure 5. The DFB grating was buried and required two regrowth stages after grating formation. There are no reported measurements on the spectral linewidths but they are expected to be in the 2 MHz range. The ex-facet power of 10 mW was improved on in a subsequent paper [23] with a value of ∼20 mW reported. The wavelength shift with current and temperature is reported to be 0.0025 nm/mA and 0.125 nm/^oC [24].

2.2.3 InP-discrete mode laser diode

Discrete mode (DM) technology is Eblana Photonics proprietary method of manufacturing single mode lasers. Single wavelength operation in DM lasers is achieved by introducing index perturbations in the form of etched features positioned at a number of sites distributed along the ridge waveguide laser cavity as shown in Figure 7. Eblana has recently fabricated single mode lasers in the 2 μm spectral region [13]. The spectral linewidths for DM laser emitting at 2.0 μm is 1 MHz. The ex-facet power of 5 mW was measured and the wavelength shift with current and temperature is reported to be 0.0025 nm/mA and 0.125 nm/°C [13].

Figure 7.
Electron micrograph of Fabry Perot laser cavity (top left) and DM laser, fabricated by etching slots into laser ridge (bottom left) along with spectral characteristics of typical mid-IR laser with and without etched features (top and bottom right respectively).

3. Mid-IR DM laser
3.1 Design of InP ridge waveguide FP lasers
A typical InP based Type-I laser structure for an emission wavelength of 1.6–2.1 μm is shown in Figure 8 [14]. For most of the laser structures reported on in Section 3, two to three compressively strained QWs are used as the active region with a width of 10 nm. They are separated by 15–30 nm thick InxGa1-xAs barrier layers which are either lattice matched to the InP substrate or tensile strained in order to reduce the average strain in the active region. The active region is sandwiched between two 200 nm-thick InGaAsP separate confinement guide layers with a bandgap wavelength of λg = 1.3 μm which also acts as an etch stop layer for ridge waveguide definition. A 1800 nm thick p-InP layer is grown on top of the separate confinement heterostructure followed by a 200 nm thick highly p-doped InxGa1-xAs contact layer [3].

For the results shown below, four laser structures with varying In compositions were grown on 75 mm diameter n-type (100)-InP substrates in a metal-organic vapour-phase epitaxy reactor at low pressure. The overlapped photoluminescence spectra for the four wafers are shown in Figure 9 measured at room temperature. The sharp peaks indicate high material quality with low defects.

The optical waveguiding properties of the ridge waveguide structure was determined by carrying out 2-D numerical simulations and one example for the 2 μm wafer (In = 0.74) is shown in Figure 10. The modal analysis of the structure showed that no higher order lateral modes are supported when the ridge width is 2 μm and that the calculated effective index for the waveguide is 3.2. This effective index is used to calculate the grating pattern spacing required to give single mode emission at the target wavelength which will be described below [3].

3.1.2 Mid-IR FP laser characterization

To evaluate the material quality before fabricating the DM lasers, broad area and FP lasers with varying cavity length were fabricated and characterised on the four wafers, only the results from the 2 μm wafer are presented in this section. As-cleaved broad area lasers with 50 μm wide ridge widths and varying cavity lengths were analysed under pulsed conditions (1 μs pulses with 0.1% duty cycle) to evaluate the material quality. The threshold current density (Jth) for the 1000 μm cavity length was 358 A/cm² (∼119 A/cm²/QW) indicating good material quality. The slope efficiency variation with cavity length allowed the internal quantum efficiency (ni) and the internal optical loss (ni) to be estimated to be 80% and 8 cm⁻¹respectively.

A 600 μm long FP ridge waveguide laser was packaged in a fiberised 14-pin butterfly module which contained a thermoelectric cooler and thermistor and the optical characteristics were measured under CW conditions. Figure 12 shows the overlapped CW measurement of light-current (LI) characteristics measured at chip temperatures 10, 25, 45, 50, 60 and 70°C. The power was measured with a large area (Ø3 mm) extended wavelength InGaAs detector (GPD 3000). The light coupling efficiency into the fibre was measured to be 60% and the chip ex-facet power was >20 mW at 200 mA, 25°C. The extracted threshold currents were 18, 30 and 58 mA at 25, 50 and 70°C respectively. The characteristic temperature of threshold current between 25–50 and 25–70°C was calculated to be 49 and 38 K. The measured slope efficiencies in the fibre at 25 and 50°C were 0.08 and 0.06 W/A respectively [3].

3.2 Design of DM laser diodes

Single wavelength operation in DM laser diodes is achieved by introducing index perturbations in the form of shallow-etched features, or slots, positioned at a number of sites distributed along the ridge waveguide as shown in Figure 16a [27, 28, 29, 30, 31, 32]. The slots are realized using ICP dry etching, with a typical depth in the region of 1.5–2 μm and a width of ∼1 μm. The slots are relatively shallow and are not etched into the active (wave guiding) region, however, they will still interact with the mode’s electric field as the mode profile is not fully confined to the active region and will expand into the surrounding cladding regions. This interaction results in a proportion of the propagating light being reflected at the boundaries between the perturbed and the unperturbed sections. In effect the slots act as reflection centres and through suitable positioning the slots manipulate the mirror loss spectrum of an FP laser so that the mirror loss of a specified mode is reduced below that of the other cavity modes [27, 28, 29, 30, 31, 32]. Using a simplified model, developed in [33, 34], a slot can be described as a one dimensional discontinuity inserted into the cavity; as most of the reflection comes from the front of the slot interface. Figure 16b shows a schematic of a laser cavity with slots introduced into the cavity; where r_s is the slot reflectivity, t_s is the slot transmission, N is the number of slots, L is the distance between the slots, and ϒ_i is the reflectivity in a section of the cavity where i is the slot number. The reflectivity from the first slot, ϒ₁ is given by r_s. The introduction of a slot into the waveguide changes its effective refractive index, so that it differs slightly from the segments of the waveguide without slots. The reflectivity from the waveguide to slot interface can be approximated using Eq. (1):

where n₁ is the effective refractive index of the waveguide and n₂ is the effective refractive index of the waveguide with a slot.

Assuming no loss from the slot Eq. (2),

ϒ₂ is the reflectivity from the second slot and is given by Eq. (3)

where β is the complex propagation constant, and the term t_s is squared to take account of forward and backward travelling waves. The exponential term describes the medium in which the light travels, and a factor of two is used again to take account of forward and backward travelling waves. The complex propagation constant takes account of the gain and loss in the transmission medium and is defined in terms of Eq. (4)

β = β re + iβ i = 2 πn λ + i g − α i 2 E4

where n is the refractive index, λ is the wavelength, g is the optical gain and α_i is the internal cavity loss. The reflectivities of the third and fourth slots are given by Eq. (5):

ϒ 3 = r s t s 4 exp − 4 iβL E5

and Eq. (6),

ϒ 4 = r s t s 6 exp − 6 iβL E6

respectively; therefore, the reflectivity obtained from four slots is given by Eq. (7):

ϒ total = ϒ 1 + ϒ 2 + ϒ 3 + ϒ 4

= r s + r s t s 2 exp − 2iβL + r s t s 4 exp − 4iβL + r s t s 6 exp − 6iβL E7

By letting Eq. (8)

X = t s 2 exp − 2 iβL E8

the total reflectivity from N slots can be expressed by Eq. (9) the following series.

ϒ total = r s 1 + X + X 2 + X 3 + . . … . + X N − 1 E9

Which in terms of known variables can be described as Eq. (10)

ϒ total = r s 1 − t s 2 exp − 2 iβL N 1 − t s 2 exp − 2 iβL E10

The power reflection is related to the reflection amplitude by Eq. (11),

R = abs ϒ total 2 E11

Using this model the power reflection versus etched feature number at a wavelength of 1887 nm was simulated and shown in Figure 17.

For a 600 μm long laser cavity at about 60 slots the peak reflectivity begins to saturate and the FWHM is about 0.9 nm which is equivalent to the FP mode spacing. So this is the optimum number of slots for this cavity length.

3.2.1 DM laser fabrication

The fabrication of the DM laser is exactly the same as the FP laser outlined in Section 3.1.2 width the addition of one extra dry etching step to etch the grating, the dry etch chemistry used again was Cl/N2 and was followed by a short wet-etch to remove surface roughness from the grating. A schematic of the etched features is shown in Figure 18.

3.2.2 DM laser diodes emitting at 1.87 μm

Low cost single mode semiconductor laser diodes emitting at 1.87 μm are highly desirable as light sources for trace gas sensing of H₂O. The measurement of H₂O is important in many industrial applications, for example, continuous emission monitoring in combustion processes where the vapour concentration can be related to performance parameters, such as, efficiency of combustion and heat release. In this section we present data on DM laser diodes operating around λ = 1.877 μm. Fabricated DM lasers exhibit continuous wave (CW) mode hop free operation in the temperature range from 15 to 55°C with emission wavelengths centred at 1.87 μm at 25°C, and ex-facet optical output power >3 mW at 25°C.

A ridge waveguide 600 μm in length DM laser diode was die bonded to an aluminium nitride submount and the optical characteristics were measured under CW conditions. Figure 19 plots the overlapped CW measurement of ex-facet LI characteristics measured at chip temperatures of 15, 25, 35, 45 and 55°C, with the same power and wavelength measurement setup as mentioned in the previous section. The extracted threshold currents were 12, 15, 18, 22 and 29 mA at 15, 25, 35, 45 and 55°C, respectively. The measured slope efficiencies at 15 and 55°C were 0.038 and 0.022 W/A, respectively.

In Figure 20 the emission spectrum of the DM laser diode is measured at a heat sink temperature of 25°C and bias current of 100 mA. A peak wavelength of 1877 nm is demonstrated, with a side mode suppression ratio (SMSR) of ∼45 dB achieved, in excellent agreement with simulated values. In Figure 21a we plot the peak wavelength versus bias current as a function of laser submount temperature. The peak wavelength tunes linearly with bias current at a tuning rate of ∼0.017 nm/mA. In Figure 21b the optical emission spectrum over a wide temperature range, from −5 to 55°C, is plotted. The single mode peak lasing wavelength shows a linear dependence with current and temperature, with a tuning rate of Δλ/ΔI ∼0.017 nm/mA and Δλ/ΔT of 0.113 nm/^oC, consistent with that expected due to the temperature-induced change in the refractive index [29].

Wide gain bandwidth is demonstrated in Figure 22 where the DM peak wavelength is varied by changing the grating period wavelength tunability of 120 nm is achieved. This makes this material promising for widely-tunable mid-infrared single-mode devices such as DM arrays, external cavity lasers and sampled gratings. Eblana’s DM technology has been used to demonstrate single frequency lasers with high side mode suppression ratios spanning a wide wavelength range from 1.75 to 2.1 μm by using the appropriate InGaAs quantum well composition and thicknesses as shown in Figure 23.

Acknowledgments
This work was supported in part by the European project MODEGAP (FP/2007-2013 under grant agreement 258033) and by the European Space Agency (ESA) under contract reference ESTEC/ITT AO/1 7204/12/NL/NA.