All-electronic frequency stabilization of a DFB laser diode

Abstract

A laser diode's junction voltage is a sensitive measure of its temperature and can be used in a thermal control feedback loop. To compensate for the temperature dependence of the laser's internal resistance, we have measured the dynamic resistance, ∂V/∂I, by modulating the injection current and measuring the demodulated voltage. The junction voltage was thus controlled while operating at fixed DC injection current. Over an external temperature range of 15°C to 35°C, this stabilised the centre frequency (wavelength) of a 1651 nm DFB laser diode with a residual mean frequency shift of 60 MHz (0.5pm), less than the uncertainty on the centre frequency of 80 MHz (0.7 pm). Under the same conditions, conventional thermistor control gave a systematic wavelength shift of -8.4 GHz (-76 pm), and control of the uncompensated forward voltage gave a shift of 9.9 GHz (90 pm).

Wavelength-locking of a semiconductor laser using an electronic technique

“…The relationship between a laser diode's junction voltage (Vj) and its operating temperature can be used as a temperature sensor and, within a control loop, to stabilise the centre wavelength of the laser diode [5] . For DFB laser diodes requiring injection currents (If) of the order of 155 mA, changes in series resistance (Rs) with temperature can create a small but significant systematic error [5] in the measured Vf. The simplified relationship between these terms is described in the equation below.…”

“…The latter is affected by the external package (case) temperature, junction heating and thermal gradients within the package. Even with good thermal package design, active wavelength stabilization is required [5,6] . If the injection current is fixed and the temperature of the laser's active region can be precisely and accurately controlled, the emitted wavelength should remain constant.…”

“…Conventional temperature control employs a thermo-electric cooler (TEC) and thermistor, the latter being located typically several millimeters from the laser's active region. This creates systematic errors [5] as the control system measures the temperature of the thermistor rather than the laser, thus is affected by changes to the temperature gradient between the thermistor and the active region, caused by changes to the external (case) temperature over the specified working temperature range. To improve the wavelength stability provided by thermistor control a number of optical techniques are currently used.…”

“…In this paper we report on the progress in developing a new wavelength stabilization technique, whereby the junction voltage (Vj) of a laser diode is used to measure the temperature of the active medium. The essence of the technique is that the series resistance of the laser diode can impose a small but significant error when measuring the forward voltage Vf, and must be compensated to recover, accurately, the underlying junction voltage [5] . Applying a sinusoidal modulation to the injection current and measuring the demodulated forward voltage allows one to measure the series resistance dynamically as δV/δI and compensate for it.…”

“…This paper further develops the technique which was described by Asmari et al and applied to a 1650nm DFB laser packaged in a TO can [5] . We have now reapplied this technique to a telecommunications DFB fiber-coupled laser and replicated the results.…”