Automatic Locking of Laser Frequency to an Absorption Peak

Langley Research Center, Hampton, Virginia

An electronic system adjusts the frequency of a tunable laser, eventually locking the frequency to a peak in the optical absorption spectrum of a gas (or of a Fabry-Perot cavity that has an absorption peak like that of a gas). This system was developed to enable precise locking of the frequency of a laser used in differential absorption LIDAR measurements of trace atmospheric gases. This system also has great commercial potential as a prototype of means for precise control of frequencies of lasers in future dense wavelength-division-multiplexing optical communications systems.

The operation of this system is completely automatic: Unlike in the operation of some prior laser-frequency-locking systems, there is ordinarily no need for a human operator to adjust the frequency manually to an initial value close enough to the peak to enable automatic locking to take over. Instead, this system also automatically performs the initial adjustment.

Figure 1. The Automatic Frequency-Locking System first sweeps the laser frequency through a wide range to find the locking range, then switches to a locking mode, in which it constantly adjusts the frequency to keep it at the middle of the locking range. (Note: PID is Proportional-Integral-Differential.)

The system (see Figure 1) is based on a concept of (1) initially modulating the laser frequency to sweep it through a spectral range that includes the desired absorption peak, (2) determining the derivative of the absorption peak with respect to the laser frequency for use as an error signal, (3) identifying the desired frequency [at the very top (which is also the middle) of the peak] as the frequency where the derivative goes to zero, and (4) thereafter keeping the frequency within a locking range and adjusting the frequency as needed to keep the derivative (the error signal) as close as possible to zero.

Figure 2. The Error Signal and its derivative are used in finding the locking range and then maintaining lock. This error signal was obtained in operation using a CO₂ absorption peak centered at a wavelength of 2,050.428 nm.

More specifically, the system utilizes the fact that in addition to a zero crossing at the top of the absorption peak, the error signal also closely approximates a straight line in the vicinity of the zero crossing (see Figure 2). This vicinity is the locking range because the linearity of the error signal in this range makes it useful as a source of feedback for a proportional + integral + derivative control scheme that constantly adjusts the frequency in an effort to drive the error to zero. When the laser frequency deviates from the midpeak value but remains within the locking range, the magnitude and sign of the error signal indicate the amount of detuning and the control circuitry adjusts the frequency by what it estimates to be the negative of this amount in an effort to bring the error to zero.

Before the laser frequency can be locked as described above, it is necessary to find the locking range. For this purpose, the frequency is swept through a broad range that includes the locking range and, in addition to determining the error signal (the first derivative of absorption with respect to frequency), the system also determines the derivative of the error signal with respect to time. The system can readily identify the locking zone because the derivative of the error signal is positive and reaches its highest value in the locking zone and is negative just outside the locking zone. Once the laser frequency is inside the locking zone, the frequency sweep is halted and the frequency-stabilization circuitry that implements the locking scheme described above is activated. In a test, the system was demonstrated to be capable of maintaining the frequency of a diode laser at the middle of a 944-nm-wavelength water-vapor absorption peak, with an error of no more than 3 percent of the full width at half maximum of the peak.

This work was done by Grady J. Koch of Langley Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Electronics/Computers category. LAR-16394-1