DE3239312A1 - DEVICE FOR SHIELDING A LIGHT SOURCE - Google Patents

Description

J.S.Heeks 35-4

Device for shielding a light source

The invention is based on a device for shielding a light source from what is reflected back to the light source Light. Such a device can be used in optical transmission systems or in optical sensor systems be used.

Semiconductor diode lasers are very sensitive to in terms of a change in their burden and therefore it is It is desirable to keep light reflected back from the optical system away from the semiconductor diode laser. It an isolator is needed that can only provide light in one direction lets through. In known devices are magnetic Effects (e.g. Faraday rotation) are used in conjunction with polarization filters, which means that im optical light path achieved a non-reciprocal behavior will. In conventional optical transmission media, one obtains however, only inadequate results. It is also difficult to implement such solutions in the technology of integrated optics. The new facility will the light source is shielded from the light reflected back by the optical system and the new device Can be realized as an integrated optical component eren.

ZT / Pi-Sm / Chr

Stuttgart, October 18, 1982

• · · α

~ 4 J.S.Heeks 35-4

The invention is illustrated by way of example with reference to the drawings explained in more detail. It shows

Fig. 1 is a sketch to explain the acousto-optical

Bragg deflection, and

Fig. 2 is a schematic representation of the new facility to shield the light source.

The new device makes use of the fact that the light that is deflected in an acousto-optical Bragg cell its frequency changes depending on the frequency of the acoustic wave. this will explained with reference to FIG. There an optical wave with the frequency f is shown, which is below a certain Angle hits an acoustic wave with the frequency f. The frequency of the deflected optical wave

is f + fg. This frequency shift is obtained when the optical wave moves "against" the acoustic wave. Only the first-order mode is taken into account here and it is possible to use the facilities like that interpret that in fact the entire input signal is too distracted from a single exit. The deflected output signal is now reflected and through the acousto-optical Deflector transmitted back, then the optical wave experiences a further frequency shift and now has the frequency f + 2f.

ο a

The device for shielding the light source, which is explained with reference to FIG. 2, has this property will, use. A semiconductor laser diode D gives one Light beam from the through a Fabry-Perot resonator R, which is tuned to the optical frequency f of the laser is going through. Downstream of the resonator is one acousto-optic Bragg cells that see an electro-optic Includes transducer T, which is at a certain angle to

J.S.Heeks 35-4

Light path is arranged. The transducer T receives an electrical signal with the frequency f _g and generates a refraction pattern across the optical light path. As a result of the refraction of the light beam, the optical output signal has the frequency f + f and the direction of the light-

O 3

After passing through the refraction field, the ray deviates from the original direction. The one by means of the refraction of light deflected light beam is focused in a known manner by means of a lens on the output 0 of the Device for shielding the light source, and at this output is the optical system to which the light beam is to be supplied connected. All light that is reflected back by the optical system, arrives again to the Bragg cell, whereby the light beam after the second passage through the Bragg cell the frequency f + 2f

ο a

Has. However, light with such a frequency is emitted by the Fabry-Perot resonator, which is tuned to the frequency f is not allowed through and thus the diode is isolated from the reflections.

The Fabry-Perot resonator, the Bragg cell and the lens can, as shown in Figure 2, be implemented as an integrated optical component. One in block B. The lithium niobate substrate becomes an optical one by means of diffusion Waveguide G made. The optical waveguide G leads to the Fabry-Perot resonator R, which was fabricated in the same surface area as the waveguide. The Fabry-Perot interferrometer is followed by a transducer T for the acoustic wave, also on the surface of the block. The converter is followed by a

3Q lens L, which is also realized by means of diffusion. The laser diode D is then attached to the end of this block, aligned with the waveguide G.

J .S. Heeks 35-4

To avoid the frequency of the laser diode itself in relation to the modulation frequency f

If the amount differs from its base frequency, the Frequency can be regulated to the frequency of the Fabry-Perot resonator. To do this is in the direction of the beam path in which there is no deflection by the Bragg cell, a photodetector diode P is provided. The output signal of the diode P is fed to a feedback L rege L loop F, which regulates the laser diode.

With this arrangement, care must be taken that not all of the optical energy is deflected but that part of the optical signal reaches the photodiode P unmodulated. Light that way The frequency that is determined by the Fabry-Perot resonator has reached the photodiode directly and thus it is possible to stabilize the laser diode via the photodetector and the feedback control loop, so that their Frequency f is always regulated to the resonance of the resonator. To illustrate the value of the new facility some numbers below:

- Semiconductor diode lasers are set to a 10 MHz long-term stability regulated with a line width of 3 MHz and a Fabry-Perot resonator of 150 MHz 3 dB resonance width.

- With the Fabry-Perot resonator the reflection coefficient is 0.97.

- When using an acoustic control signal with the frequency of 500 MHz, the reflected optical wave is offset by 1000 MHz and this has the consequence that an isolation of 20 dB takes place in the above resonator.

Claims (7)

3 2393Ύ2 INTERNATIONAL STANDARD ELECTRIC CORPORATION, NEW YORK J.S.Heeks 35-4 claims 1. Device for shielding a light source from for Light source reflected back light, thereby gekennzei chnet that in the beam path of the light beam emitted by the light source an acousto-optical isolator with an acousto-optical Bragg cell is provided, and that between the Bragg cell and the light source (D) an optical filter (R), which on the Frequency of the light source is matched, is present. 2. Device according to claim 1, characterized in that that the optical filter is a Fabry-Perot resonator. 3. Device according to claim 1 or claim 2, characterized in that the Bragg cell has a device (T) for surface acoustic waves, which on the Surface of an optically transparent piezoelectric Substrate is realized. 4. Device according to one of claims 1 to 3, characterized characterized in that the optical filter and the Bragg cell by a single integrated optical component ment is realized in a Li thi utnniobat substrate. 5. Device according to claim 4, characterized in that that in the lithium niobate substrate there is still a focus ZT / Pi-Sm / Chr Stuttgart, October 18, 1982 32393Ί2 J.S.Heeks 35-4 sizing optics is realized, namely in the light beam path after the optical filter and the Bragg cell. 6. Device according to one of claims 1 to 5, characterized in that a control device (F) is provided is that part of the light beam that does not have a deflection has learned, is supplied (P), and which the frequency regulates the light source so that it remains the same as the frequency to which the filter is tuned. 7. Device according to one of claims 1 to 6, characterized characterized in that the light source is a semiconductor laser is.