Page 383 - Fiber Bragg Gratings

360 Chapter 8 Fiber Grating Lasers and Amplifiers

Figure 8.2: The chirped external fiber grating cavity laser [17]. The special
AK-coated hyperbolic lens on the end of the fiber provides high-efficiency coupling
of >98% [19], which in turn allows stable single-mode operation over a large
output power (15 mW).

The principle of operation of chirped grating is to compensate for the
current induced change in the optical cavity length of the semiconductor.
As the length of the semiconductor cavity changes by Sl s to maintain an
integral number of periods within the laser cavity, the wavelength shifts,
altering the cavity length. In principle, the wavelength should shift
smoothly as the bias current is altered.
In this case, the grating had a deliberate chirp of 0.28 nm with
decreasing wavelengths away from the gain medium, a reflectivity of 33%,
and a grating length of 10 mm. The fiber lens had a hyperbolic shape to
maximize coupling to the grating. The coupling efficiency of this type of
lens can be as high as 98% [19]. With an HR coating on the rear and an
All coating on the output facet, the threshold of the device was measured
to be 7 mA, operating single-frequency at fiber output powers of >15 mW.
A side-mode suppression ratio (SMSR) of 56 dB was also demonstrated
at a bias current of 250 mA(~25 mW output power). The linewidth was
measured to be between 100 and 540 kHz (close to a mode hop), depending
on the bias current.
An important application of the fiber Bragg grating is for stabilization
of erbium amplifier pump lasers operating at 980 nm [20,21]. Since the
pump absorption band is narrow, it is useful to maintain the pump wave-
length accurately. It was found empirically that placing a 4% reflector
reflection grating approximately 50-100 mm away from the AR-coated
facet of a high-power pump-laser chip caused it to lock to the grating.
Temperature variations of 50°C pulled the wavelength by —0.2 nm, show-
ing that the requirement for a thermoelectric cooler may not be necessary.