HU193436B - Selective optical sensing apparatus, mainly for optical telecommunication equipments and optical locators - Google Patents

Abstract

A selective optical detector for detecting a substantially punctual light susceptible of being collected in a predetermined wavelength range and visual angle is provided with an optical collector element made of a light-transparent material having wavelengths comprised within a predetermined range and a light-sensitive detector element. The invention is characterized in that the optical collector system has a focus surface characteristic of the predetermined wavelength range and which is located at a certain distance from the focus surfaces which are characteristic of the wavelengths outside of said range. The detector element is optically connected to the optical collector system by an opening provided in the focus surface pertaining to the predetermined wavelength range. The size of the opening substantially corresponds to the size of the surface of the focus of the light coming from the predetermined visual angle and pertaining to the predetermined wavelength range.

Description

BACKGROUND OF THE INVENTION The present invention relates to a selective optical sensing apparatus suitable for receiving optical signals in applications where the spatially transmitted beam is parallel or can be made and is substantially monochromatic. The sensor should only detect light in the narrow wavelength range and be insensitive to light outside the range.

Significant worldwide research is underway to extend the operating frequency of radar, telemetry, communication, and other specialty measuring devices to the optical range.

In the case of optoelectronic devices and measuring devices that receive direct or reflected signals of low intensity in space (cosmic, atmospheric, underwater), the receiver's sensitivity to the background and interfering light is significantly reduced. The background and distracting light from the receiver's angle of view make it impossible to take advantage of the sensitivity of the electronic components used in the receiver. Therefore, night and daytime performance (range) is often given separately for the above mentioned equipment. The receiver of such optoelectronic devices usually has the function of receiving a narrow spectrum optical signal. The sensing element is usually a photon multiplier or some photosensitive semiconductor device capable of receiving a relatively wide optical spectrum without having its own selectivity. In the prior art optoelectronic devices, the effects of background and interference are reduced by narrowing the receiver's field of view and applying selective or color filters.

The disadvantages of this solution are: the receiver's field of view cannot be reduced indefinitely due to technical-technological constraints (targeting and recording stability), and partly due to the inhomogeneity of the transmission medium and beam migration. In optoelectronic devices, the selection of the spectrum to be sensed and the reduction of background and interfering light are performed using filters. The low bandwidth interference filter with a spectral width of about 5 to 20 nm provides relatively good selection. These interference filters are used in receivers of several prior art optoelectronic devices. Their common disadvantage is the additional damping introduced by their application. There are many examples of such applications in the literature.

B.G. King., P.J. Fitzgerald and H.A. The optical receiver designed for the communication experiment described in Steint: An Experimental Study of Atmospheric Optical Transmission (The Bell System Technical Journal Vol. 62, No. 3, 1983) uses a narrowband filter which causes a loss of 3 dB. In addition to the filter, there are several ways to reduce the effect of the backlight, shielding cassettes are placed in front of the optics, and the receiver's field of view is limited.

MJ Green: Application of an Optical Data Link in an Airborne Scanning System (Review of Scientific Instruments, pp. 538, 1278-1280, 1982), which does not result in high gain in the optical receiver, which excludes visible light employed.

G.Michael Lauham: In the optical system of the Air Force Lasercom Space Measurement Unit (CH 1939-6 / 80, 1980 IEEE 27.2.1-27.2.3), the receiver includes a narrowband (interference) filter.

Thomas F.Wiener, in his Strategic Laser Communications article (CH 1539-6 / 80 IEEE 27.4.1 - 27.4.5), mentions the use of appropriate filters as a fundamental problem for the outdoor optical receiver and suggests a solution - a complex crystalline physical filter - for installation. A sketch of such a filter is described in the September 1979 issue of Electronic Design, "EÖTF Independently Controls Wavelenghth and Linewidth".

However, the use of interference filters and resonant narrowband filters operating on a similar principle raises a number of problems, the most important of which are:

- the interference filters are made using sophisticated technology and require a high degree of precision, which makes them expensive and difficult to mass-produce,

- in addition to the relative gain from the selectivity of the interference filters, significant losses are expected as the useful signal is also greatly attenuated, thus reducing the receiver sensitivity (constant range) when permanently installed in the equipment without background and interference light , although at that time your filter-to-noise ratio would not limit it,

- the integration of the interference filters into the receiver-side optical system is simple, since its filtering effect is mainly applied to an axial beam parallel to the optical axis of the system, while beams arriving at different angles to the axis give resonance at different wavelengths. The correction of the error is possible only by adding additional optical elements and increasing the number of optical elements. However, the associated increase in the number of optical interfaces will result in additional losses,

- the interference filters are made at a fixed wavelength and cannot be tuned so that the receiver cannot be adjusted to the wavelength of the light to be detected. The transmitted wavelength range is the result of technology which cannot be changed after incorporation into the optical system, but at 2193436 the wavelength of light emitted by the light sources used as the transmitter is scattered, e.g. shifts for writing objects.

Due to the limited capabilities of the interference filters, other solutions are sought, e.g. using dual refractive crystals, filters that can be tuned by acoustic wave or electric field have been developed which are more complex than interference filters and require significant additional attenuation.

Because of the problems listed above, some of the more modest outdoor optoelectronic devices do not use interference filters or other high-selectivity optical filters, but only filters with significantly lower selectivity, which, due to a lack of significant selectivity, do not increase optical signal-to-noise ratio. , only to a certain extent protect the sensor device from the thermal effects of extremely high intensity background and interfering light. Eg Sándor Takács (BME) In the telecommunication assembly described in his article Broadband Communications Experiments in the Optical Range (Telecommunications, 1980, p. 350) no protective devices are used to exclude ambient light. The solution used in the optical receiver described in Liechtenstein Patent No. CH 625923 does not protect against interference from the backlight.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a selective optical sensing device for use mainly in optical communications equipment and optical locators.

- it does not attenuate light received over a specified wavelength range beyond the attenuation of the optical acquisition system,

- allows selection and detection of a useful signal of less than an order of magnitude in background and interfering light to achieve adequate reception,

- allows the detection equipment to be tuned to the desired wavelength,

- allows selection bandwidth to be set,

- allows to adjust the reception characteristic of the detection device.

The object of the present invention is achieved by the apparatus shown in Figures 1 and 2. The selective optical sensing device is particularly suitable for optical communication devices and optical locators for detecting low-bandwidth or monochromatic light beams in which the optical acquisition system and the sensor element C having a light-sensitive surface are encapsulated in an optically closed low-reflection case , in which case the light-sensitive surface of the sensor element C is arranged behind the sealing surface B so that it is in the path of light passing through its optical opening F. The focal length wavelength function of the optical acquisition system A is monotonically variable within the limits of the range of the wavelength to be detected, and the optical spot F is equal to or greater than the focal area F of the detected wavelength between the light sensitive surface of the optical sensor system. The optical aperture F formed on the locking surface B is disposed in the plane of the focal point f corresponding to the wavelength of the detected light.

Figure 2 is a graph showing the attenuation wavelength characteristics of the selective optical device shown in Figure 1.

The notations in Figure 1 also mean f, f ₂ , f focal length, and aa the main plane of the optical acquisition system A.

Optical acquisition system A has different focal points f ', f, f' for light of different wavelengths due to color difference of optical acquisition system A, but due to spherical error of optical acquisition system A or other distortions, In this case, the filtering effect is only effective with the additional damping. Of course, depending on the intensity distribution of the focus spot, selectivity can be increased at the cost of additional attenuation. The optical aperture F on the closure surface B is located at a point of imaging corresponding to the wavelength of light to be detected. The sensor element C is located in the path of the light to be detected, behind the sealing surface B. The light may be provided with a light guide, e.g. can be deflected by fiber optics.

Preferably, the optical aperture F, together with the sensor element C, or, in the case of a sufficiently large sensor surface, without the sensor element C, is movable according to the orbit of the imaging point as a function of wavelength, so that the system can be tuned. In the case of light coming from several directions, an optical aperture F may be associated with each direction.

It may be advantageous if the size of the optical opening F is variable, since by changing the size the bandwidth can be adjusted.

It may be advantageous if the shape of the optical aperture F is variable, since different bandwidth characteristics can be obtained for light coming from different directions according to the shape. This solution is particularly advantageous if the transmission path dispersion or the receiver mechanical vibrations have a preferred direction.

If your assistants are unlinked or attached, you may also need to use the optional H mirror. With this solution, the same Optical Collection System can be used for example. targeting and / or bidirectional linking is also possible.

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It may be advantageous to have an additional covering element G between the optical collection system A and the sealing surface B with the optical opening F, since this prevents the non-fractured beam entering the shaft from passing through the opening without filtering effect. This measure increases the page slope of the sensing element C bar. Such an additional cover element may also be located in front of the optical collection system A.

The sensor element C of the device according to the invention is preferably an optoelectric converter.

The invention will now be described in more detail with reference to the drawing, which illustrates an exemplary embodiment of an optical sensor according to the invention.

Figure 1 is a sectional view of the selective optical sensor apparatus A according to the invention with distorted dimensions for the sake of illustration.

Figure 2 shows the damping wavelength characteristic of the sensor of Figure 1.

Figure 1 illustrates the optical acquisition system A - in practice, generally a lens system. Optical acquisition system A is preferably sized for large color deviation but small distortion optics, having an optical aperture F with an aperture F in the plane of focus for the wavelength to be detected, and a detector element C behind the aperture B. The sensor element C is a known optoelectric converter e.g. PIN diode. A beam of light of the selected wavelength passes through the optical aperture F of the locking surface B in the focal plane of the wavelength of light to be sensed, without damping, and reaches the surface of the sensor element C. This is represented by the radius D of the receive wavelength D in FIG. Other wavelengths have a different focal point f, the focal point f, f ', f' being shifted as a function of the wavelength on the optical axis b-b, resulting in a larger light spot in the plane of the closing surface B than the optical aperture F on the closing surface. This is represented by the radius of this non-reception wavelength in FIG. The attenuation characteristic of the detector element C according to the invention is calculated as a function of the wavelength in relation to the size of the optical aperture F and the magnitude of the light spot formed in the plane of the optical aperture F.

As an example, Figure 2 shows the damping characteristic of such a tried and tested optical collection system. The Optical Collection System used is a three lens assembly carefully corrected for spherical error. As a result of proper correction, the focal spot resulting from a spherical error is 20 pm. The lenses are made of SF6 type heavy-duty fiberglass with significant color dispersion. The optical collecting system A used has a focal length of 80 mm and a luminance of 1. The change in slope of the focal length wavelength function 4 at 920 nm is 9.4 pm / nm. AB sealing surface applied ^- F optical aperture diameter in the case of curve I 50 pm to 300 pm, the curve II. From the data of the example, it can be seen that, without the addition of additional damping, significant and at the same time wide range of selectivity can be achieved by realistic means using the present invention.

The selective optical sensing apparatus of the present invention has been used in a receiver of an optical communication apparatus developed for digital transmission. The receiver is designed to receive an optical signal of 820 nm wavelength. A semiconductor laser was used as a near monochromatic source in the transmitter. The avalanche photodiode used in the typical slope of 65 A / W, zajekvivalens optical output 3 · 10 ^'15 W / Hz ^1/2. The effective surface area of the sensor element is 0.2 mm ² . As a result of the insertion of optical barrier B in combination with the exclusion of internal reflection and scattering and the wavelength-dependent transmission due to color deviation and small optical aperture F, the optoelectronic communication device equipped with the sensor element C according to the invention has a daily range of increased overnight without altering other characteristics, its night range (no background and no distracting operation) remained unchanged, leaving the original 10-15 km.

The same results are expected for other active or passive optoelectronic devices based on the effects of low bandwidth primary or secondary signals (laser rangefinders, laser locators, passive infra-locators, active infrared telescopes).

In summary, the selective optical sensing device of the present invention has a number of advantages which include:

- eliminates the need for interference filters or more complex crystalline physics filters,

- its use results in a net profit, does not result in additional attenuation,

- it is simple and cheap to produce and does not require any special technology,

- tunable to the wavelength to be detected,

- adjustable bandwidth,

- the directional characteristics of the optoelectronic device are variable,

- significantly increases the range of outdoor optoelectronic devices.

Claims (8)

A selective optical sensing device, in particular for optical communication devices and optical locators for low-bandwidth or monochromatic light-beam arrays 4193436, comprising an optical acquisition system (A) directed toward the light beam to be detected and a sensing element (C). ) is arranged in an optically sealed, low-reflection case, in which the light-sensitive surface of the sensing element (C) is arranged behind a sealing surface (B) so that it passes in the path of light through its optical opening (F), (A) Focal length 10 - its wavelength function within the range of the wavelength to be detected is monotonically variable between the optical acquisition system (A) and the light-sensitive surface of the sensor element (C) with the size of the focal spot of the wavelength to be detected s, the optical aperture (F) formed on the closure surface (B) is disposed in the plane of the focal point (f) of the wavelength of the detected light. Selective optical sensing device according to Claim 1, characterized in that the optical aperture (F) is arranged movably along the path of the focal point 25 (f) as a function of the wavelength of light to be detected. Selective optical sensing device according to claim 1 or 2, characterized in that the size of the optical opening (F) is variable. 4. A selective optical sensing device according to any one of claims 1 to 3, characterized in that the shape of the optical opening (F) is variable. 5. A selective optical sensing device according to any one of claims 1 to 3, characterized in that it has an additional mirror and / or a coupling mirror. 6. A selective optical sensing device according to any one of claims 1 to 3, characterized in that an additional covering element (G) is disposed between the optical collection system (A) and the sealing surface (B) provided with the optical opening (F). 7. A selective optical sensing device according to any one of claims 1 to 5, characterized in that an additional cover element (G) is provided in front of the optical collection system (A). 8. A selective optical sensing device according to any one of claims 1 to 5, characterized in that the sensor element (C) is an optoelectric converter.