Note: Some of the graphical elements of this site are only visible to browsers that support accepted web standards. The content of this site is, however, accessible to any browser or Internet device.
Tunable Diode-Pumped Microchip Laser
The longitudinal mode spacing of a laser depends upon the length of the laser cavity. If the length of the cavity is extremely short (of the order of a few hundred microns), the mode spacing will be large (~100 GHz). If the gain linewidth of the laser is narrow, this ensures that only one cavity mode can lase, and thus the laser is single frequency. In a material with a wide gain bandwidth, such as Cr:LiSAF, the cavity length required for single frequency operation is less than 10 µm, which is too small to be practical. An alternative is shown in figure 1, where cavity lengths on the order of a millimeter are used, but with a coupled 'composite cavity' consisting of two gain regions.
![[Microchip laser schematic]](images/mc-cavity.gif)
Figure 1: Composite cavity Cr:LiSAF microchip laser
With this scheme, interferometric mode selection between the multiple etalons of the coated surfaces ensures that the laser operates at a single frequency. An advantage of this technique is that the laser may be tuned by altering the separation of the two gain crystals. Due to the vernier-type coupling between the etalons, only a small movement is neccesary to produce a wide frequency shift, and so a piezoelectic actuator could be used.
The microchip laser could be pumped either by a krypton-ion laser or a AlGaInP diode laser. The diode-pumped device showed superior stability, due to the greater intensity stability of the pump source. Approximately 1 mW of single frequency output was emitted, tunable from 847 nm to 875 nm.
![[Spectrum of laser output]](images/mc-spect.gif)
Figure 2: Spectrum of the single frequency laser on linear and logarithmic scales
Single frequency operation of the device is shown in figure 2, with no other modes detectable down to a level 50 dB below the peak level. A scanning Fabry-Perot linewidth analyser showed the output of the laser had a linewidth of less than 20 MHz, which was the resolution limit of the analyser.
Relevant publications
J. M. Sutherland, S. Ruan, R. Mellish, P. M. W. French and J. R. Taylor: 'Diode-pumped, single-frequency, Cr:LiSAF coupled-cavity microchip laser', Optics Communications, 113, pp.458-462 (1995)