1. Introduction

Reductions of cost and power consumption of 1.3 μm optical transmitter modules are required for wide spread implementation of medium distance, high density optical networks. Significant cost advantages are offered by semiconductor lasers that can be operated uncooled at high bit rates over a wide temperature range. InP based lasers, using InAlGaAs material, have demonstrated uncooled operation at high bit rates and temperatures, although the variation of the threshold current with temperature is relatively high with a characteristic temperature (T ₀) of around 100 K [1]– [2]. Therefore, GaInNAs lasers on GaAs substrates have been considered for further improvements of the temperature stability due to the larger conduction band offset offered by the GaInNAs/GaAs system [3]. The use of multiple quantum wells (MQWs) is imperative for good temperature stability and high speed operation. Using a laser with three GaInNAs QWs, Dagens et. al. demonstrated uncooled 2.5 Gb/s operation up to a temperature of 85 °C at an emission wavelength of 1.35 μm [4]. However, strain accumulation in GaInNAs MQWs with a high In-content and a low N-content limits the number of QWs and minimizing the number of QWs is therefore important for high material quality and low threshold and operating currents [5]. Here we show that, using only two GaInNAs QWs with GaAs barriers, lasers with low threshold current, high T ₀, and emitting at 1270-1300 nm can be operated uncooled at 2.5 Gb/s in the temperature range 25-110 °C. Under conditions of constant modulation voltage and only the bias current adjusted for constant average output power, an extinction ratio of more than 6 dB and a spectral rms-width less than 2 nm are obtained.

2. Laser design and fabrication

The epitaxial material was grown on an n ⁺-GaAs substrate by molecular beam epitaxy using a load-lock RF nitrogen plasma source for the growth of GaInNAs. The undoped active region consists of two 7-nm-thick GaInNAs QWs (with an In-content of 38.7%), separated by a 20-nm-thick GaAs barrier layer and embedded between 20-nm-thick GaAs layers. It is surrounded by 160-nm-thick graded composition Al_xGa_1-xAs (x=0.2-0.5) layers and doped Al_0.5Ga_0.5As cladding layers. The details of the structure and growth conditions can be found in Refs. [6]–[7]. Broad area lasers (100 μm wide and 1000 μm long) fabricated from this material have a threshold current density as low as 300 A/cm² (150 A/cm² per QW). Ridge waveguide (RWG) lasers with a ridge width of 3 μm were fabricated in an industrial process at Modulight Inc. and cleaved into bars with cavity lengths of 250, 300, 500, and 750 μm. The used chip design and process were not optimized for operation under high-speed modulation. Facet coatings were deposited, resulting in facet reflectivities of approximately 30 and 70%. Separate laser dies were subsequently soldered p-side up on AlN sub-mounts for temperature dependent static and high frequency measurement. Note that the present lasers have no thick dielectric layers under the bond pad for reducing the parasitic capacitance, resulting in a very simple fabrication process.

3. Temperature dependent static characteristics

The output power (from the low reflectivity facet) and voltage were measured as a function of current under continuous operation at different temperatures. Results for 250 and 500 μm long RWG lasers, in the temperature range 25–110 °C, are shown in Fig. 1. The 250 μm long laser (Fig. 1(a)) has a low threshold current of 11 mA at room temperature, increasing to 15 mA at 85 °C and further increasing to 23 mA at 110 °C. Corresponding values for the slope efficiency are 0.42, 0.34, and 0.25 W/A. The maximum output power at thermal roll-over is 40 mW at room temperature and 9 mW at 110 °C. The differential resistance is 10 Ω and is more or less temperature independent. The emission wavelength red-shifts with temperature, from 1270 nm at room temperature to 1300 nm at 110 °C. Clearly resolved longitudinal modes were observed at all temperatures and currents, indicating stable fundamental transverse mode operation. The beam divergence angles (full-width-at-half-maximum) were measured to be 27 and 55 degrees in the horizontal and vertical directions, respectively.

 

Fig. 1. Output power and voltage as a function of current in the temperature range 25-110 °C for (a) a 250 μm and (b) a 500 μm long RWG laser.

Download Full Size | PDF

The dependence of threshold current on temperature for RWG lasers of different length is shown in Fig. 2. Below a certain critical temperature, the lasers have high values of T ₀, being as high as 200 K for the 250 and 300 μm long lasers and reducing to 164 K for the 750 μm long laser. Above this critical temperature the threshold current increases faster with temperature and T ₀ is reduced. The critical temperature is clearly cavity length dependent and increases from 75 °C for the 250 μm long laser to 105 °C for the 750 μm long laser. This indicates that the critical temperature may be related to an onset of carrier leakage out of the QWs [8] since longer cavity lasers operate at a lower carrier density and therefore a higher temperature is needed for a substantial amount of thermionic emission. Auger recombination may also contribute at high temperatures [9]. Adding a third QW could improve the high temperature performance through reduced leakage and Auger recombination at lower carrier density.

 

Fig. 2. Threshold current as a function of temperature for RWG lasers of different length. Values of the characteristic temperature below the critical temperature are also indicated.

Download Full Size | PDF

The low threshold current, the high slope efficiency, and their small temperature dependencies provide favorable conditions for high speed modulation over a wide temperature range with low power consumption.

4. Temperature dependent modulation characteristics

Dynamic measurements were performed by directly probing the laser with a 50 Ω high frequency microwave probe (Picoprobe 40A). The bias current and the high frequency signal were combined in a bias-T and fed to the laser through the probe. The output from the laser was coupled to a single mode fiber with a conical tip to minimize optical feedback.

The small signal modulation bandwidth was determined by S₂₁-measurements using a 20 GHz network analyzer (Agilent N5230A) and a 15 GHz optical receiver (HP 11982A). The 250 μm long laser had a 3-dB-bandwidth of 3 GHz, limited by the large bonding pad capacitance of the used, unoptimized chip design. On the other hand, measurements on a 400 μm long RWG laser with a 4 μm ridge width and a thick layer of benzocyclobutene (BCB) under the bond pad, fabricated from the same material, revealed a bandwidth as high as 14 GHz, limited by thermal effects [10], and an intrinsic damping limited bandwidth of 25 GHz. Therefore, at room temperature the bandwidth of the present lasers is largely limited by the capacitance which is relatively insensitive to temperature variations.

Studies of the 2.5 Gb/s modulation response of the 250 μm long laser were performed by measuring eye-diagrams using a non-return-to-zero pseudorandom bit sequence of 2⁷-1 word length from a pattern generator (HP 70843A) and a 15 GHz optical receiver (HP 11982A) followed by a 2.4 GHz Bessel filter and a digital communication analyzer (Agilent 83480A). Systematic variations of bias current, modulation voltage, and ambient temperature were used to map the dependencies.

Figure 3 shows filtered eye diagrams recorded at temperatures of 25, 85, and 110 °C with a constant modulation voltage of only 0.5 V and the bias current adjusted (21, 28, and 39 mA) for constant average output power at a level where the extinction ratio (ER) exceeds 6 dB. The use of a constant modulation voltage over such a large temperature range reduces the demands on the drive circuitry. Clear open eyes, which should allow for error free transmission at distances of interest for access and enterprize network applications, are observed at all temperatures. It is also observed that the quality of the eye improves with temperature through a decrease of the rise time. This is due to the reduction of the laser resonance frequency with temperature which, at the highest temperature, partly compensates the parasitic roll-off and therefore increases the bandwidth somewhat.

 

Fig. 3. Eye diagrams recorded at 25, 85, and 110 °C with a constant modulation voltage of 0.5 V and an extinction ratio of 6 dB. Corresponding bias currents are 21, 28, and 39 mA.

Download Full Size | PDF

To further relax the requirements on the drive circuitry, we also investigated the 2.5 Gb/s modulation response under conditions of constant modulation voltage and bias current. Results with a bias current of 35 mA and a modulation voltage of 0.5 V are shown in Fig. 4. Again, clear open eyes are observed but with a temperature dependent ER of 2.2, 3.3, and 8.7 dB at 25, 85, and 110 °C, respectively. At the highest temperature, the off-level approaches the threshold current and some turn-on delay and increased timing jitter are observed. If restricting the maximum operating temperature to 85 °C, a higher ER could be obtained at constant modulation voltage and bias current with a higher modulation voltage of 1.0 V. This is shown in Fig. 5 where the ER is 4.6 dB at 25 °C and 8.3 dB at 85 °C.

Since the spectral width of the emission under modulation limits the maximum transmission distance when single mode fibers are used, we also determined the spectral rms-width by recording emission spectra under the modulation conditions used in Fig. 3. The spectral rms-width was calculated according to

 

Fig. 4. Eye diagrams recorded at 25, 85, and 110 °C with a constant modulation voltage of 0.5 V and a constant bias current of 35 mA. Corresponding extinction ratios are 2.2, 3.3, and 8.7 dB.

Download Full Size | PDF

where λ₀ is the mean wavelength and p_i is the power at a particular wavelength λ_i in the spectrum. Values of 0.6, 1.4, and 2.0 nm were found at 25, 85, and 110 °C, respectively. The increase of the spectral width with temperature is related to the broadening of the gain spectrum through the temperature dependent Fermi-distribution. Using a standard single mode fiber this would allow for a dispersion limited transmission distance at 2.5 Gb/s of several kilometers at all temperatures (the calculated maximum distance is 30 km for a Corning SMF-28 fiber).

The same set of measurements were also performed on 500 μm long lasers and similar performance could be obtained although a slightly higher bias current and modulation voltage had to be used due to the higher threshold current and the lower slope efficiency.

5. Conclusions

Using high quality GaInNAs/GaAs double quantum well laser material we have fabricated facet coated 1.3μm ridge waveguide lasers having a combination of low threshold current, high slope efficiency, and small temperature dependencies of both threshold current and slope efficiency below a certain critical temperature. Short cavity lasers have a characteristic temperature of the threshold current as high as 200 K. The simple laser design, without thick dielectric layers under the bond pad, limits the modulation bandwidth to 3 GHz as a result of the parasitic capacitance. The temperature stability allows for uncooled 2.5 Gb/s operation in the temperature range 25–110 °C. The small temperature dependencies of the slope efficiency and the differential resistance allow for a constant modulation voltage over the entire temperature range and by only adjusting the bias current for constant average output power an extinction ratio of more than 6 dB can be obtained. Under the same modulation conditions the spectral rms-width is below 2 nm. With clear open eyes the spectral rms-width should allow for error free transmission at 2.5 Gb/s over several kilometers of standard single mode fiber.

 

Fig. 5. Eye diagrams recorded at 25 and 85 °C with a constant modulation voltage of 1.0 V and a constant bias current of 35 mA. Corresponding extinction ratios are 4.6 and 8.3 dB.

Download Full Size | PDF