GB2109122A

GB2109122A - Acousto-optic isolator

Info

Publication number: GB2109122A
Application number: GB08133054A
Authority: GB
Inventors: John Stuart Heeks; John David Jackson
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1981-11-03
Filing date: 1981-11-03
Publication date: 1983-05-25
Also published as: GB2109122B; AU553871B2; IT1237333B; AU8985282A; JPS58140717A; JPS6357768B2; DE3239312A1; IT8224032A0

Abstract

An acousto-optic isolator for a semiconductor laser diode D comprises a Fabry-Perot resonator R tuned to the laser optical frequency f0 followed by an acousto-optic Bragg diffraction device utilising surface acoustic waves of frequency fa launched from the transducer T. Light focussed by lens L to the load via output 0 has a resultant frequency f0 + fa. Any light reflected from the load suffers a further change in frequency to f0 + 2fa in the diffraction device and hence is blocked by the resonator. Undiffracted light can be detected to photo-detector P and used in a feedback control loop F to stabilise the light source D to the resonator. <IMAGE>

Description

SPECIFICATION Acousto-optic isolator This invention relates to an acousto-optic isolator for use in optical transmission systems or optical sensor systems.

Semiconductor diode iasers are sensitive to changes in their output loading and it is therefore desirable to protect them from optical power reflected from the system. Hence the need for some sort of unidirectional optical isolator. Traditionally magnetic effects, such as Faraday rotation, are used in conjunction with polarisation filters to establish nonreciprocal behaviour in the optical path, but this mechanism is weak in normally used optical transmission media. Moreover, integrated optical solutions using this technique appear unlikely.

According to the present invention there is provided an acousto-optic isolator comprising an acousto-optic Bragg diffraction device in the output path of a monochromatic light source and, interposed between the device and the source of an optical filter tuned to the optical frequency of the source.

The invention also provides a method of isolating a semiconductor diode light source from changes in the output loading comprising the steps of passing the diode output through an optical filter tuned to the diode output frequency and then through an acousto-optic Bragg diffraction means whereby the output light frequency is changed and any light reflected from the load has its frequency further changed by the diffraction means so as to be blocked by the filter.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates the acousto-optic Bragg diffraction mechanism, and Figure 2 illustrates an acousto-optic isolator.

The present invention is based on the fact that an optical wave undergoing an acoustooptic Bragg diffraction has its optical frequency changed by the acoustic frequency.

This mechanism is illustrated in Fig. 1 which shows how an optical wave of frequency f, incident at an angle to an acoustic wave of frequency fa has its optical frequency changed. As shown, if the optical wave is moving "against" the acoustic wave the optical frequency f, is translated to fro + f,. Only the first order mode is involved and it is possible to design devices where virtually all the input signal is diffracted into a single output. If now the diffracted output is reflected and retransmitted through the acousto-optic diffraction device the optical wave suffers a further change in frequency and is now at frequency fO + 2fa The acousto-optic isolator shown in Fig. 2 makes use of this double change of frequency.Light from a semiconductor diode laser D is transmitted through a Fabry-Perot resonator R which is tuned to the optical frequency f0 of the laser. Following the resonator there is an acousto-optic Bragg diffraction device incorporating an electro-optic transducer T set at an angle to the optical path from the resonator. The transducer T is energised with an electrical signal of fre quency fa so that a diffracting pattern is set up angularly across the optical path. Resulting from the diffraction mechanism an optical output of frequency fO + fa is obtained, the resultant optical path diverging from the original path. The diffracted wave is conveniently focussed by a lens for onward transmission via output 0 to whatever system the laser diode is powering.Any light reflected from the system passes back along the optical path to the diffraction device where it suffers a second change in frequency to f0 + 2fa . Light at this optical frequency will be rejected by the Fabry-Perot resonator tuned to fO, thus isolating the diode from the reflections.

The Fabry-Perot resonator, the Bragg diffraction device and the lens are conveniently fabricated as an integrated optics device as shown in Fig. 2. A block B of lithium niobate has an optical waveguide structure G diffused into one surface region, leading to a Fabry Perot resonator R fabricated in the same surface region. Following the resonator an interdigitated surface acoustic wave transducer T is deposited on the surface of the block. Beyond the transducer a lens L is fabricated by diffusion processes. The laser diode D is then affixed to the end of the block in alignment with the guide G.

However, in order to ensure that the laser diode frequency does itself not deviate by an appreciable amount compared with impressed modulation fa the diode must be stabilised to the same Fabry-Perot resonator. To achieve this a photodetector diode P is placed on the line of the original optical path and connected by a feedback control loop F to the laser diode drive circuitry. In this configuration it is deliberately arranged that not all the optical signal is diffracted in order that some unmodulated signal can fall on photodiode P. Light passing along this path has impressed on it the frequency discrimination characteristic of the Fabry-Perot resonator and hence, via the photodetector and the feedback control, the laser diode can be stabilised so that its frequency f0 is pulled to the natural resonant frequency of the resonator.

To illustrate the practicality of this invention it is worth noting the following facts:- (1) Semiconductor diode lasers have been stabilised to -10 MHz long term stability with 3 MHz line width using a Fabry Perot cell of 1 50 MHz 3 dB resonant width.

(ii) A mirror reflection coefficient of .97 is appropriate to the above Fabry- Perot response.

(iii) Using a 500 MHz acoustic driving signal the reflected optical wave is displaced by 1000 MHz and this would result in an excess of 20 dB isolation in the above resonator.

Claims

1. An acousto-optic isolator comprising an acousto-optic Bragg diffraction device in the output path of a monochromatic light source and, interposed between the device and the source an optical filter tuned to the optical frequency of the source.

2. An isolator according to claim 1 wherein said optical filter is a Fabry-Perot resonator.

3. An isolator according to claim 1 or 2 wherein said Bragg diffraction device comprises surface acoustic wave structure formed on the surface of an optically transparent piezo-electric body.

4. An isolator according to any preceding claim wherein said optical filter and said Bragg diffraction device are together fabricated as an integrated optics structure in a body of lithium niobate.

5. An isolator according to claim 4 including a focussing lens arrangement in the lithium niobate which has passed through the filter and diffraction device.

6. An isolator according to any preceding claim including means for detecting undiffracted light from the light source and feedback control means to the light source to stabilise the light source frequency with respect to the filter.

7. An isolator according to any preceding claim including as a light source a semiconductor diode laser.

8. An acousto-optic isolator substantially as described with reference to Fig. 2 of the accompanying drawings.

9. A method of isolating a semiconductor diode light source from changes in the output loading comprising the steps of passing the diode output through an optical filter tuned to the diode output frequency and then through an acousto-optic Bragg diffraction means whereby the output light frequency is changed and any light reflected from the load has its frequency further changed by the diffraction means so as to be blocked by the filter.

1 0. A method of isolating a semiconductor laser diode substantially as described here inbefore.