Deterministic wavelength tunability is a key advantage of Photodigm's monolithic DBR lasers. Applications requiring the precision of distributed Bragg reflector laser diodes are becoming more and more demanding in terms of power and mode purity. In pulsed applications, wavelength agility, stability, and peak power are critical performance parameters. Spectroscopic analysis requires a wavelength scan across the spectral feature to resolve it. The narrow bandwidth and tunability of the DBR laser enables extremely high resolution over a narrow portion of the spectrum, opening up spectroscopic applications not available to conventional slit based spectrometers. Furthermore, by repetitively scanning across the feature, an error signal can be generated that allows locking of the laser output to a specific transition.
The wavelength tuning requirements for DBR lasers vary from one application to another. Therefore, understanding of the operation of key elements of the device is needed to ensure that the desired performance is achieved. In the Photodigm DBR, the key elements are the gain section and the DBR section. Tuning of a DBR laser is accomplished by either change of injection current into the gain section, or through change in temperature of the entire device by altering the heatsink temperature. The change of the refractive index with temperature is the dominant tuning mechanism. Changes in carrier density have smaller effects on the lasing wavelength. Physical elongation of the laser cavity with temperature also contributes to wavelength tuning, but is a much smaller effect than the other two effects.
Figure 1. Cross sectional view of a simplified DBR laser structure.
Figure 1 shows the cross-sectional view of a simplified DBR laser structure. The structure consists of a gain section of length Lg and a passive DBR section of length LDBR. When current is injected into the laser structure, the gain section will experience an increase in temperature, which is proportional to the dissipated power Pdiss and the thermal resistance RT between the junction and the heat sink. For simplicity we will assume that the heat flow is primarily in the vertical direction and the temperature of the DBR grating section is equal to the heatsink temperature and is unaffected by the injected current itself. In practice a very small temperature increase in the DBR section, especially at higher injection currents is observed, but this does not have a significant on the output wavelength.
An increase in temperature of the gain section will alter the refractive index of each epitaxial layer and therefore will shift the cavity modes towards longer wavelengths. At a constant current, increasing temperature will red shift the laser output. At constant temperature, increasing the current will red shift the laser output.
The mode-hop-free current range is the minimum required current change necessary to shift the wavelength equal to the amount of a mode spacing determined by the effective cavity length. Figure 2 shows the red shift with increasing current. Mod hops occur at deterministic points where the DBR selects a more efficient operating point at a lower wavelength.
Figure 2. Measured emission wavelength against injection current at different heatsink temperatures. The width of the continuous wavelength tuning decreases with increasing injection current (indicated by ∆1 and ∆2).
Furthermore the wavelength change is linear in injection current, which again can be seen in Fig. 2 at different temperatures. Figure 2 shows that for a fixed temperature the DBR laser emits in a single spectral mode with several wavelength discontinuities, with the distance between them decreasing with increasing injection current. These wavelength discontinuities are associated with an abrupt wavelength change, which correspond to the value for the longitudinal mode spacing. In general, the mode closest to the DBR peak reflectivity has the lowest modal loss and hence becomes the lasing mode. As the injection current and therefore the temperature increases, the modes shift to longer wavelengths, but at a much faster rate than the temperature dependent DBR reflectivity spectrum. As the lasing mode moves away from the DBR peak reflectivity, there comes a point where the adjacent mode has a higher reflectivity. At this point the wavelength suddenly changes to a different cavity mode, resulting in a decrease of the emission wavelength.
Changing the heatsink temperature will not only shift the cavity modes, but will also shift the reflectivity curve of the DBR. The wavelength shifts in the two regions are equal if both sections experience the same temperature change. However, the increase in temperature reduces the optical efficiency and leads to an additional heating in the gain section. However, the additional heating due to injection current is much less than the effect of changing the heatsink temperature. To a first order, heating due to injection current can be neglected. The wavelength shift with temperature is almost independent of the injection current and amounts to 0.074 nm/K, as is shown in Figure 3.
Figure 3. Measured emission wavelength against heatsink temperature at different injection currents. The individual curves are offset for clarity by 0.3 nm in the vertical direction.
In general, the change in the dissipated power is much larger with current than with temperature, i.e. (in units of Ampere and Kelvin), which leads to a larger mode-hop-free tuning range by changing the temperature than by changing the injection current. For typical test devices in this work, we measured a change in mode spacing with current, ∆Im, of 0.05 A and a change in modal spacing with temperature, ∆Tm , of 75°K, which results in a ratio of 1 : 1500. Note the exact values may vary with the specific operation conditions.
Figure 4. Contour plot of the emission wavelength against injection current and heatsink temperature. The two solid lines indicate a mode-hop-free constant power operation with a tuning range of 2 and 1.7 nm, at an output power of 60 and 90 mW, respectively.
Figure 4 shows a contour plot of the emission wavelength as function of the injection current and heatsink temperature. It can be clearly seen that the largest mode-hop-free tuning can be achieved with temperature. For applications requiring a constant output power, a mode-hop-free temperature tuning range of over 2 nm can easily be achieved (as indicated by the two bold solid lines at two different power levels in Fig. 4). In contrast, the wavelength shift with current amounts to less than 0.15 nm corresponding to a change in injection current of roughly 50 mA.
In summary, wavelength tuning in DBR laser diodes consists two dominant mechanisms -- one driven thermally, and one driven by current. The thermal tuning is constant and amounts to 0.06 nm/K and is dependent on the design wavelength and operating optical power. In general, mode hop free tuning can easily be achieved over several tens of degrees. In contrast, current tuning is associated with repeated mode jumps separated in wavelength by the mode spacing, whereas the latter is determined by the gain section length and the effective length of the DBR grating. Between mode jumps the wavelength shifts quasi linearly with an average amount of approximately 0.0025 nm/mA, depending on the operating conditions. For applications requiring a large mode-hop-free wavelength tuning range the DBR laser is best operated at a fixed injection current value and the heatsink temperature should be used to tune the wavelength.