Tunable diode lasers (TDLs) are versatile, wavelength-agile, solid-state analytical light sources that are especially useful in a variety of spectroscopy applications. When driven with a linear current ramp, a TDL-based spectrometer can sweep its powerful monochromatic beam across the absorption spectrum of a sample gas mixture, producing a sensitive and detailed analysis of the mixture. TDLs can do this because their instantaneous emission wavelength is controlled (that is, tuned) by the amplitude of the applied drive current.
Unfortunately, a TDL’s emission wavelength is also strongly influenced by temperature. Consequently, TDL applications must incorporate active, high-stability, and often complex temperature regulation. Even then, temperature stability can be a problem, particularly in industrial and aerospace applications, where ambient conditions can vary widely. One of the most intractable of those problems involves the effects of thermal gradients when a discrete temperature sensor (for example, a thermistor) is used to sense the temperature of the TDL.1
The solution presented here avoids this problem by letting the TDL self-sense its own temperature. Self-sensing is possible because, as in other forward-biased diodes, the TDL’s forward voltage depends on device temperature (Fig. 1). So, theoretically at least, sensing the TDL’s temperature is as simple as sensing its forward voltage: V(t).
Practically, however, complications arise. First, V(t) varies not just with temperature but also with drive current, I(t), and TDL spectroscopy usually involves modulation of the diode drive. V(t) therefore has an ac component that must be filtered out. In addition, V(t) consists of not only the TDL forward voltage, but also an ohmic component from all the resistances (for example, of interconnecting wiring) in series with the TDL. These resistances can vary with ambient temperature, and their contribution to the sensor signal must therefore somehow be removed to ensure accurate sensing of TDL temperature.
The solution to both problems is differential-gated integrate-and-hold circuitry (Fig. 2). The linear ramp of the TDL’s drive current, I(t), induces a proportional linear ramp V(t) component. The designer can modify the software of the spectrometer microcontroller to produce a new logic signal (Centroid Logic) accurately timed to switch at the centroid (equal-area) point in the drive-current waveform and to control CMOS polarity switches S1 and S2. This arrangement causes the integral of V(t) over the interval t to T to be subtracted from the integral over 0 to t.
Because the timing of the Centroid Logic signal is such that the integrals of I(t) over these periods are equal, the integrals of the corresponding V(t) are also equal, and therefore they exactly cancel out. The result is a diode forward voltage, Vf, output signal that accurately equals the TDL forward voltage, completely independent of ohmic components. The synchronous integration feature cancels ripple and, if the R and C values are chosen as shown in Figure 2, integration settling occurs in a single cycle time, T. The principle of quick-settling, ripple-free gated integrate and hold was discussed in an earlier article.2
The Vf output is ideal for duty as a temperature sensor input to a typical TDL thermoelectric cooler thermostat. The output’s immunity to thermal gradient effects provides superior temperature stability and therefore superior emission wavelength control in challenging environments.