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New Schemes for Frequency Stabilization of Lasers Using Fabry Perot Filters

IP.com Disclosure Number: IPCOM000036458D
Original Publication Date: 1989-Oct-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 3 page(s) / 58K

Publishing Venue

IBM

Related People

Green, PE: AUTHOR [+2]

Abstract

Two schemes for locking an amplitude-modulated laser to a tooth of a Fabry Perot interferometer are disclosed. These schemes can be extended to lock a set of lasers to different teeth of a Fabry Perot. Background

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New Schemes for Frequency Stabilization of Lasers Using Fabry Perot Filters

Two schemes for locking an amplitude-modulated laser to a tooth of a Fabry Perot interferometer are disclosed. These schemes can be extended to lock a set of lasers to different teeth of a Fabry Perot. Background

Lasers tend to drift in frequency over time for several reasons, such as changes in ambient temperature. In a communication network using wavelength division multiple access (WDMA) 1, several lasers operating at different wavelengths are used over the same fiber to

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provide multiple non-interfering channels. Hence, the lasers must be stabilized to prevent them from drifting into each other. The amount of stabilization required depends on the channel spacing and the type of detection used. In a WDMA application, it is desirable to lock a set of lasers relative to each other.

A few solutions to this problem have been proposed. Two of these 2,3 solutions use Fabry Perot (FP) filters. In the first 2, the lasers are passed through a FP whose passband is scanned over the entire frequency range of interest, and the output optical spectrum is observed. By looking at the location of the peaks and comparing them with the desired locations, appropriate control signals can be generated for each laser.

The second approach 3 was proposed to stabilize FSK-modulated lasers. Here, the lasers are locked to the teeth of a FP. Consider one particular laser. The FP passband is chosen to span the frequency range over which the laser is FSK modulated. The output of the FP is passed to a photo-detector that generates a current proportional to the input optical intensity. When the laser is perfectly locked, the 0s and 1s are both passed identically by the FP and the measured photo- current is simply a DC. When the laser begins to drift in one direction, depending on the direction, either a 0 is passed more than a 1 or vice- versa. This is reflected in the photo-current which looks either like the modulating FSK bitstream or its inverse. A control signal can be generated by correlating the photo-current with the sent data signal.

In many applications, the laser is on-off (intensity) modulated. Two solutions for stabilizing intensity-modulated lasers are disclosed. Stabilization Schemes

A laser is passed through a FP, one of whose passbands is centered at the laser frequency. The output is passed through a photo-detector, and the photo- current is observed. As the laser begins to drift from the center of the passband, the received intensity decreases and this is reflected in the photo-current. However, a means for determining the direction of drift must also be incorporated, and the two schemes below differ in the implementation of this feature. Proposed Solution 1

The output of the laser is split and passed to two FP filters, as sh...