KR100286066B1 - Optical and optical scanning devices and optical information recording devices - Google Patents

Abstract

An optical device 2 is described in which electromagnetic radiation is augmented with frequency. The device 2 comprises a diode laser 12 and a nonlinear optical medium 31 for increasing the frequency due to electromagnetic radiation, which causes the radiation frequency emitted by the diode laser 12 to be increased. It plays a role. The diode laser 12 is a pulsed diode laser, and feedback means for setting the radiation emitted by the diode laser to the wavelength within the allowable band of the medium is arranged in the optical device 2. Feedback can be realized optoelectronically or optically.

Also described is an apparatus for optically scanning an information plane that includes an optical device, such as a radiation source unit.

Description

Optical and optical scanning devices and optical information recording devices

1 is a schematic diagram of an apparatus comprising an optical device according to the invention and for optically scanning an information plane.

2 shows a first embodiment of an optical device according to the invention in which photoelectric feedback occurs.

3 shows a second embodiment of an optical device according to the invention in which photoelectric feedback occurs.

4 shows a first embodiment of an optical device according to the invention in which photoelectric feedback occurs.

5A is a diode laser spectral diagram of a device in which wavelength selective feedback is used.

5b is a diode laser spectral diagram of the same device without feedback.

6 shows a pulse train emitted by the diode laser.

7 shows an embodiment of a planar grating suitable for use in the optical device according to the invention.

8-15 illustrate a number of different embodiments of an optical device according to the present invention in which optical feedback occurs.

16 and 17 show an embodiment of two optical devices according to the present invention in which photoelectric and optical feedback are combined.

18 to 20 show an embodiment of the optical device according to the invention in which the frequency amplified radiation intensity is switched.

21A-21C show an embodiment of an optically transparent body in the form of a planar-parallel plate for folding the light path of an optical device according to the invention.

22A and 22B are plan and side views showing portions of a second embodiment of an optically transparent folding body for folding an optical path in three dimensions in an optical device according to the invention.

23 shows a third embodiment of an optically transparent body in the form of a bar for folding the optical path of an optical device according to the invention.

<Explanation of symbols for main parts of drawing>

1: Optical scanning device 3: Optical recording medium

5: transparent substrate 7: reflective information layer

9: track 13: radiation source unit

15: radiation beam 19: detection system

25: motor 27: arrow

28 beam splitter 31 nonlinear optical medium

37: active control system 39: detector

The present invention relates to an optical device in which electromagnetic radiation is frequency boosted, the device comprising a diode laser for supplying radiation to be frequency boosted and a nonlinear optical medium having an allowable bandwidth, wherein the frequency boost is ensured by the nonlinear optical medium. .

The invention also relates to an apparatus for optically scanning an information plane including such a device.

Tolerant bandwidth is the width of the wavelength band near the nominal wavelength of radiation, which can be efficiently increased by nonlinear optical media.

Scanning means, for example, scanning while recording an information plane of an optical recording medium and scanning while reading an information plane.

The device of the type described at the outset is described on page 48 in the publication "Electronics" published in August 1988 under the heading "Blue-light laser ups CD density". The device described in the above publication is used in an apparatus for reading an optical recording medium in which an audio program is stored. By doubling the frequency, ie halving the radiation wavelength of a conventional diode laser, the diameter of the read point formed by the radiation can be halved. Therefore, it is possible to read the details of the information whose size is half of the readable information without doubling the frequency. Frequency doubling has the great advantage that the information density in the optical recording medium can be greatly increased, for example, by a factor of four.

In order to improve the efficiency of the frequency boost, a relatively long nonlinear optical medium should be used. However, most media have a relatively small allowable bandwidth to impose relatively stringent requirements on the diode lasers of such devices. The most important requirements are as follows.

1) The wavelength band of the radiation emitted by the diode laser must be within the allowable bandwidth of the nonlinear optical material.

This requirement limits the output of a usable diode laser to a significant range.

2) The diode laser must have a very stable emission wavelength such that the emission wavelength is always within the allowable bandwidth of the nonlinear optical medium. This means that the output spectrum of the diode laser should not change.

In particular, since diode lasers and nonlinear optical media have high temperature dependent properties, the latter requirement can be difficult to realize because the diode laser and the nonlinear optical media have to be stabilized, for example, with high temperature accuracy up to 0.5 ° C.

If the temperature deviation causes a change in the output spectrum of the diode laser, the device becomes inefficient by emitting different wavelengths and emitting radiation from the nonlinear optical medium that has virtually never doubled or increased frequency.

The present invention aims to provide a device of the type described at the outset, wherein the device is significantly less sensitive to temperature variations, and the minimum required amount of radiation with increasing frequency remains unchanged and the amount of radiation mentioned last is considerably A means that can be increased.

There are a plurality of embodiments of the device according to the present invention, in which the diode laser is a pulsed diode laser and feedback means for setting the radiation supplied by the diode laser to a wavelength within the allowable bandwidth of the nonlinear optical medium is provided. It has common features.

The combination of the feedback means and the diode laser emitting pulsed radiation offers the possibility of correcting the desired diode laser wavelength with small deviations or affecting the noise characteristics so that the radiation with an augmentation frequency is invariably obtained.

A first embodiment of a device with this aspect is provided with an active control system in which a feedback means comprises a detector and a control unit, the detector being sensitive to frequency boost radiation, wherein the control unit is at least one that determines the frequency boost radiation. It is characterized by being controlled by the output signal of the detector affecting the parameter of.

This embodiment takes advantage of the fact that the pulsed diode laser is a multimode laser and emits radiation at different wavelengths. Diode laser radiation is multiplied in frequency when at least one mode is within the allowable bandwidth of the nonlinear optical medium or when the average wavelength of two modes present simultaneously is within the allowable bandwidth. In these cases, frequency boost radiation can always be detected. Based on this detection, the amount of frequency boost radiation can be affected by feedback.

Frequency-increasing pulsed diode lasers are described in the Sept. 15, 1979, journal Applied Physics Letters 30 (6), titled "Second-harmonic generation with Ga _1-x Al _x As lasers and KNbO ₃ crystals." It is well known in the paper.

However, this paper describes only experimental data on the determination of the effect of temperature change on frequency multiplication by the combination of diode laser and nonlinear optical media. Pulsed diode lasers are used because these pulsed diode lasers can provide high power. The paper does not describe the recognition that different wavelengths supplied by such diode lasers are usefully available. The article has a significantly reduced temperature sensitivity, but in contrast does not describe a practically applicable device in which a diode laser and a nonlinear optical medium are arranged in one oven.

Since the power in a pulsed diode laser is distributed in a plurality of modes, for example 10, the power of frequency multiplied radiation is in principle a device with a mono mode diode laser that emits continuously if the device is very well stabilized in terms of temperature. Can be obtained from However, this power loss is limited, as described above, because radiation in two laser modes where the average frequency is within the allowable bandwidth of the nonlinear optical medium can be doubled in frequency when the two laser modes are present at the same time. As is known, the power per pulse of a pulsed diode laser is also significantly higher than the continuous power of a mono mode diode laser.

According to the second aspect of the present invention, the power of the frequency boost radiation can be greatly increased by using optical feedback instead of photoelectric feedback.

A second embodiment of the device with this aspect is characterized in that the feedback means are formed as optical wavelength selective feedback means.

The radiation portion of the diode laser radiation whose wavelength is within the allowable bandwidth of the nonlinear optical medium is fed back by the feedback means. As a result, the pulsed diode laser acts as a mono mode laser, the radiation of which is all suitable for increasing in terms of frequency.

In the second embodiment, the frequency-increased radiation amount can be significantly increased compared to the frequency-increased radiation amount obtained through the first embodiment. As one advantage, the single mode can be selected in another way by varying the diode laser current and diode laser temperature. The second embodiment is characterized in that the wavelength selective feedback means comprises feedback means for at least partially reflecting arranged at a distance d from the diode laser. The distance d satisfies the following condition.

d = c / 2 nT-c / 2 ε (p + Δp) where p is the pulse duration of the emitted laser pulse, T is the pulse period, n = an integer, and c is the speed of light in the medium through which the radiation beam passes , p is the formation time of the pulse LP in the diode laser, and ε is the real number satisfying 0 &lt; ε &lt; In decreasing, when this radiation pulse enters the diode laser, the following condition is satisfied.

E (Pr)〉 E (LPi)

Where E (LPi) is the radiant energy produced by the diode laser.

Due to this measure, the diode laser is not affected by the reflection of the laser radiation on the optical element of the device.

It is generally known that the diode laser is sensitive to laser light returning to the active layer of the diode laser via reflection in the radiation path of the laser beam. Depending on the amount of radiation returned, this can lead to undesirable effects such as changes in the output spectrum due to increased line width, high noise or variations in laser wavelength. The behavior of a pulsed diode laser is largely determined by the events occurring within the laser at the time intervals at which new light pulses occur, and because such surplus photons reach a sufficiently large number of surplus magneto magnets as a result of accurate feedback, It is known that the behavior of the laser is mainly determined by adapting the energy of the reflected radiation pulse and its delay time. As a result, the diode laser can be controlled in a prescribed manner by the installed feedback, which results in significantly lower values for pulses which cause the radiation pulse to reach the diode laser at the same moment as the result or the result of the installed feedback and which do not affect the diode laser. By having strength, the effects of other unwanted feedback can be reduced.

The feedback element may be arranged on the same side as the diode laser as the nonlinear optical element. The distance d is the distance between the feedback element and the exit face of the diode laser. Radiation emitted from the back of the diode laser can also be used to stabilize. At this time, the distance d is the distance between the surface and the feedback element. In view of the design efficiency and the degree of freedom for the optical device, the latter is preferred.

An alternative embodiment of the device according to the invention is characterized in that the wavelength selective feedback means comprises a grating.

The grating is a very suitable wavelength selection device.

Another embodiment of the device according to the invention is characterized in that the grating is a holographic grating.

Another embodiment of the device according to the invention is characterized in that the grating is a planar grating present in the fiber and continuous to the medium.

An alternative embodiment of the device according to the invention is characterized in that the grating is a planar grating arranged in a medium.

The advantage of the two embodiments described last is that the grating is not a dividing component of the device, but rather forms part of a component already combined with the device.

A device in which the wavelength selective feedback means has a reflecting element partly at the above-mentioned distance from the diode laser may have a split reflecting element and a split wavelength selecting element. However, the devices are characterized in that they are all wavelength selective feedback means formed from one element which partially reflects and selects the wavelength.

By combining two functions into one device, partial reflection and wavelength selection, the number of components can be reduced, saving space.

This embodiment of the device according to the invention may have the additional property that the device is an etalon.

Etalons can be very suitable elements to realize wavelength selective reflection. Etalon means an element having a surface that abuts two partially reflecting planes or curved faces arranged at a predetermined distance from each other. Some medium, such as air, may be present between the two sides. Etalons may alternatively be photoconductive fibers or optical waveguides having a predetermined length.

Alternatively, the last mentioned embodiment is characterized in that the device is a grating partially reflected.

An additional embodiment of the optical device according to the invention is characterized in that folding means for folding the radiation path are arranged between the diode laser and the feedback means.

The distance d between the feedback element and the diode laser necessary to stabilize the spectrum of the laser beam can then be realized in a small volume so that the device is compact.

The folding means may, for example, consist of two opposing reflective surfaces between the light beams through which the light beams propagate.

A preferred embodiment of the optical device according to the invention consists of a folding body of optically transparent material, the folding means having at least two reflective surfaces and provided with an entrance window and an exit window, It is characterized in that one of the four sides is provided with a third window for transmitting diode laser radiation to / from the feedback means.

The means for folding the optical path preferably consists of a body formed of one piece. In this way the tolerance is determined only when the body is manufactured, rather than by the mutual positioning of the two reflecting surfaces as in the case of the two opposing facing surfaces.

The folding body can be made of glass with a relatively high refractive index, for example n = 1.8. The geometric path length can then be reduced by a factor of 1.8 for the geometric path length in air with n = 1 so that the device will be more compact.

Both the inlet and outlet windows can be provided in the first surface of the folding body and occupy the same space at the same time. In this case, the diode laser radiation and the frequency boost radiation must be separated from each other between the place where the frequency boost occurs and the folding body. This may be possible for example by means of a wavelength selecting element such as a beam splitter so that the frequency boost radiation may be coupled outside the device.

Alternatively, the inlet window may be provided in the first reflective surface and the outlet window may be provided in the second reflective surface. In this case, the frequency augmentation radiation need not be coupled outside the device by the extra element, and the exit window can be formed with a partially transmissive reflector.

The partially transmissive reflector is preferably a wavelength selective reflector that reflects diode laser radiation and thus transmits frequency amplified radiation.

The optical device according to the invention is preferably characterized in that the feedback means are integrated in the third window.

Due to this combination, the number of components can be reduced.

A possible embodiment of the optical device according to the invention is characterized in that each reflecting surface is provided with a layer having a high reflection coefficient.

Due to the layer with high reflection coefficient, the intensity loss in the folded light path is limited.

When the folding body is behind a frequency boosting medium, this embodiment is characterized in that the high reflecting layer preferably has a higher reflection coefficient for the reflection supplied by the diode laser and a lower reflection coefficient for the frequency boost radiation.

A possible embodiment of the optical device according to the invention is characterized in that the folding body is a plane-parallel plate in which the first reflecting surface and the second reflecting surface are opposite and parallel to each other.

Another embodiment of the optical device according to the invention is not required for the body to which the highly reflective layers are folded, wherein the folding body is present in a medium having a refractive index smaller than the refractive index of the body material, the folding body reflecting the internal radiation totally ( totally and internally reflect) and at least one or more surfaces, and the radiation is reflected at least once by each of the two surfaces when traversing a coplanar radiation path within the folding body.

The optical device also has an optical prism arranged in the entry and exit windows, the surface of the prism traversing the chief ray of the beam, through which the radiation beam enters and exits the prism.

Light prisms prevent false reflections when the radiation beam enters and exits the folding body.

In another embodiment of the optical device according to the invention, one of the reflective surfaces is provided with a fourth window, which is arranged with a retro-directive element, leading to the first through the many reflections to the reflective surfaces. After traversing the radiation path, radiation is trapped in the first plane within the folding body, reflected parallel to itself, and reenter the body, thereby returning to the reflective surfaces through much reflection in the plane parallel to the first plane. Leading to at least one or more second radiation paths.

The reverse element is characterized in that the incident radiation beam propagating through the body along the first radiation path in a first plane perpendicular to the first and second reflective surfaces is directed to the body in a second radiation path in a second plane parallel to the first plane. It is guaranteed to be converted into a reflected radiation beam propagating through. In this way the third dimension of the body is also used to fold the radial path. The reverse element may for example be arranged on the folding body as a prism with an apex angle of 90 °. Alternatively, the prism may be installed directly in the plane-parallel plate.

Such a shape may be repeated several times so that two or more parallel planes are available.

Another embodiment of the optical device according to the invention, wherein the feedback means comprises a grating, is characterized in that the grating extends at a small angle other than 0 ° with respect to the third window.

Since the wavelength resolution of the grating is also determined by the diameter of the beam incident on the grating, this resolution can be improved by causing the beam to enter the grating at a larger angle and increasing the diameter of the radiation spot on the grating. .

Another embodiment of the optical device according to the invention is that the folded body is rotatably arranged over a small angle with respect to the radiation beam supplied by the diode laser to change the wavelength of the radiation beam reflected towards the diode laser. It features.

It is possible to select a given wavelength by orienting the body and consequently orienting the wavelength selective element integrated into the body in a different manner with respect to the incident radiation beam.

Another embodiment of the device according to the invention comprises a detector that is sensitive to the frequency boost radiation and a control unit that is adjusted by an output signal of the detector to affect at least one parameter that determines the amount of frequency boost radiation. The optical wavelength selection feedback means is supplemented by an active control system.

By adding an active control system to the optical feedback means it is possible to allow for a wider range of temperature changes and to tune diode laser wavelengths and the allowable bandwidth of nonlinear optical media with much more precision.

Depending on the choice of wavelength selection element, the preferences on the affected parameters change.

The embodiment using an additional active control system is characterized in that the parameter is a current through a diode laser and the control unit controls this current.

Alternatively, this embodiment is characterized in that the parameter is a diode laser temperature and the control unit controls this temperature.

In both cases, the operation of the diode laser is affected and the laser wavelength can be accurately corrected. By changing the laser current or laser temperature the output spectrum will shift. Since the grating absolutely limits the wavelength and the etalon periodically defines the wavelength in combination with the output spectrum of the diode laser, such control is more effective in the device using the etalon than in the device using the grating. In devices using a grating, it is possible to incorporate sophisticated control within the mode distance in the output spectrum of the diode laser, while in devices using etalons it can be stabilized in other modes.

Embodiments using an additional active control system are characterized in that the parameter is the temperature of the nonlinear optical medium and the control unit controls the temperature.

An embodiment of such a device is characterized in that the parameter is the reflectivity of the nonlinear optical medium and the control unit controls the magnitude of the electric field for this medium.

The two control devices allow the allowable band of nonlinear optical media to be shifted, which is suitable for devices using gratings as well as devices using etalons. The advantage of optoelectronic control over temperature control is the high rate of adaptation.

Another embodiment of the device according to the invention is characterized in that the diode laser is a self-pulsing diode laser.

Some pulsed diode lasers are suitable for such applications. Automatic pulse generating lasers are known first of all in British Patent Application No. GB 2 221 094.

Another embodiment of the device according to the invention is characterized in that the nonlinear optical medium comprises a nonlinear optical crystal.

The choice between these two possibilities is also determined by the requirements placed on the device and the price of the device. The advantage of the crystal is that the bond is simpler and more mechanically stable than in the case of waveguides. On the other hand, in the case of waveguides, the return efficiency of diode laser radiation can be higher.

An additional embodiment of the device according to the invention is characterized in that the waveguide is formed of one of KTP, LiNbO ₃ or LiTaO ₃ .

KTP (potassium titanate), KiNbO ₃ (lithium niobate) and LiTaO ₃ (lithium tantalum) are suitable materials for increasing the frequency.

An additional embodiment of the device according to the invention is characterized in that the nonlinear photonic crystal is KNbO ₃ or KLiNbO ₃ .

In the crystalline form, LNbO ₃ (potassium niobium) and KLiNbO ₃ (potassium lithium niobium) are well suited as nonlinear minerals to increase the frequency.

The invention also relates to an apparatus for optically scanning an information plane, the apparatus having an optical system and a radiation source unit for focusing the radiation given by the radiation source unit at a scan point in the information plane. do.

The apparatus further comprises a radiation sensitive detection system suitable for reading and writing record carriers and for returning radiation of the information surface to an electrical signal, wherein the radiation source unit comprises an optical device as previously mentioned. It is characterized by.

In the apparatus, which apparatus is particularly suitable for writing a record carrier and comprises an intensity switch for switching the intensity of the beam provided by the radiation source unit in accordance with the information recorded, the radiation source unit is referred to above. Device, the strength switch having the following parameters:

The repetition frequency parameter of the diode laser pulses,

The optical path length parameter of the radiation pulse returning to the diode laser,

The energy parameter of the radiation pulse returning to the diode laser, and

Means for setting one of the allowed band parameters of the nonlinear optical mediator.

FIG. 1 shows an apparatus 1 for optically scanning the information surface of the optical recording medium 3. The term scanning is interpreted to mean scanning during recording as well as scanning during reading of the recording medium. The recording medium 3 partially shown in the radiation cross section is formed of a transparent substrate 5 and a reflective information layer 7. This information layer 7 has a plurality of information areas (not shown), which are optically distinguished from their surroundings. The information area is arranged on a plurality of tracks 9, for example quasi concentric tracks which collectively form spiral tracks. For this purpose, the apparatus comprises a radiation source unit 13 for supplying a radiation beam 15, an optical system 17 for connecting the radiation beam 15 to a scanning point 11 on a recording medium 3, and a recording. An optical device 2 having a detection system 19 for converting radiation 21 reflected from the medium 3 into an electrical signal is provided. The beam 15 generated at the radiation source 12 and radiated at the radiation source unit 13 is a read point 11 in the information plane that reflects the beam 15 by the objective lens system 16 shown as a single lens element. Is connected to. When the recording medium 3 is rotated by the shaft 23 driven by the motor 25, the information track is scanned. By moving the recording medium 3 and the device 2 with respect to each other in the direction of the arrow 27, the entire information surface can be scanned.

The beam 21 reflected during the scanning changes in intensity depending on the information stored in the continuous information area. In order to separate the projected beam 15 from the emitted beam 21, the optical system 17 can be provided, for example, with a partially transparent mirror (not shown), which mirrors the reflected and varied beam ( A portion of 21 is reflected back to the radiation sensitive detection system 19. However, it is suitable to use a combination of the polarization sensitive beam splitter 28 and the λ / 4 plate 30 shown in FIG. This allows the laser beam 15 to have a polarization direction that can all be passed by the beam splitter 28. In the beam path of the recording medium 3 this beam first traverses the λ / 4 plate 30 and after reflection by the recording medium the second traverses the λ / 4 plate 30 to the beam splitter 28. The polarization direction is rotated 90 ° before it is incident again. Thus, beam 21 is wholly reflected back to detection system 19.

For a detailed description of the reading device see no. Of the journal Philips Technisch Tijdschrift 40 by M. G. carasso, I. B. H. Peek and J. P. Sinjou. 9, 1981/82, p. 267-272, in a paper entitled "Het systeem" compact Disc Digital Audio.

In order to miniaturize such a device, a diode laser is selected as the radiation source 12. In addition, such an apparatus requires a small scanning point because the information density of the optical recording medium can be increased to a considerable extent. The scanning point can be reduced as desired by doubling the radiation frequency of a conventional diode laser or halving the wavelength. Frequency doubling can be realized by passing diode laser radiation through a nonlinear optical medium. One of the characteristic parameters of nonlinear optical media is the allowed bandwidth. This is a wavelength band, and within this band, it is possible to reliably double the frequency of radiation supplied with the optical medium. However, the area of the tolerance band is highly dependent on temperature. At a given temperature, the crystal determines at which wavelength the frequency multiple contributes to the radiation.

On the other hand, diode lasers have the disadvantage that the wavelength is also highly dependent on temperature. Diode lasers are also very sensitive to laser light returning to the active layer of the laser due to reflection in the optical system. Thus, not only the intensity of the laser beam but also the output spectrum of the laser can be significantly affected.

In order to generate sufficiently doubled frequency radiation, the laser wavelength and region of the allowable band of the optical medium must be tuned and tuned to each other. This can be achieved by stabilizing diode lasers and nonlinear optical media very accurately with temperature, which is a relatively difficult and expensive solution. According to the present invention, this problem can be realized quite simply by using a pulsed diode laser jointly with feedback means. This can ensure that diode laser radiation is set to a wavelength within the allowable bandwidth of the nonlinear optical medium.

Desired limitations can be realized in several ways. The first method is the optoelectronic method shown in FIGS. 2 and 3. In this case, we take advantage of the fact that the pulse diode laser's spectrum consists of several modes, so that there are several wavelengths, at least one of which is invariant within the allowable bandwidth and two of these wavelengths are averaged within the allowable bandwidth. Has a wavelength. Therefore, the frequency multiplied radiation amount can be detected even at a relatively large temperature change, and by this radiation, the diode laser wavelength can be optimally set through feedback.

The radiation beam 15 generated by the diode laser 12 in the radiated unit 13 shown in FIG. 2 is multiplied by a radiation frequency, for example by a first lens system 29 consisting of two lens elements. The paper is connected in the nonlinear optical medium 31. The radiation 33 emitted from the medium 31 is connected at the detector 39 in response to the increased frequency, for example by the second lens system 35 composed of a single lens element. This detector forms part of an active control system 37 which also has a comparator 41 and a control unit 43. Some of the frequency multiplied radiation is converted into an electrical signal S _D by the detector 39. This signal is compared in the comparator with a reference signal that represents the desired amount of frequency multiplied radiation. The difference between the signals S _D and S _ref is converted into the control signal S _c , and the control unit 43 is controlled by this signal S _c . The radiation wavelength supplied by the diode laser 12 with respect to the detector output signal S _D , which is a measure of the frequency multiplied radiation intensity, can be set by changing one of the adjustable parameters of the diode laser. Depending on the parameters that can affect the amount of frequency multiplied radiation, the control unit 43 can be realized differently. Adjustable parameters are, for example, laser current and laser temperature. This means that the control unit 43 can be modified to set the current or diode laser temperature. The choice between the two alternatives is indicated by I / T in the figures. Since the laser wavelength changes with temperature, the wavelength that supplies the maximum power of frequency multiplied radiation can be set accurately by changing the temperature of the diode laser.

3 is another embodiment of the optoelectronic feedback in which the control unit 43 affects nonlinear minerals. Such control can be, for example, temperature control. By varying the temperature of the nonlinear mineral, the allowable band of the nonlinear mineral can be shifted to optimally overlap with the wavelength band of the diode laser.

Another optoelectronic feedback is the provision of electrodes to nonlinear minerals. By supplying a voltage between the electrodes and supplying an electric field to the nonlinear optical medium in accordance with the detector signal S _D , the refractive index of the medium can be changed so that the tolerance band can be shifted.

Optoelectronic feedback can also be realized dynamically. To this end, the intensity of the frequency boost radiation is modulated with a small amplitude and phase locked detection of the radiation is utilized. That is, the phase of the modulation signal S _D is compared with the phase of the control signal to which modulation is established. In this way, small changes in the frequency boost radiation intensity can be detected earlier and corrected. The intensity of the frequency boost radiation can be modulated, in particular, by changing the DC component of the current through the diode laser or by electro-optically changing the allowable band of the nonlinear medium.

In order to control the maximum of the frequency boost radiation by means of the optoelectronic feedback 37, only a small part of the radiation is used so that the remainder is a useful radiation for use in an optical device such as the scanning device shown in FIG. do. Useful radiation RU is in accordance with FIGS. 2 and 3 by using a partially transparent element such as a partially transparent mirror 44 disposed at an arbitrary position between the nonlinear optical element 31 and the detector 39. Can be separated from the device.

Instead of lens systems 29 and 35 for focusing the radiation emitted by the device by illuminating the exit face of the diode laser on the nonlinear optical device, optical fibers or planar waveguides may alternatively be used.

A significant increase in the intensity of the frequency boost radiation can be obtained by realizing optical feedback, which is an optional wavelength instead of optoelectronic feedback.

4 shows an embodiment of the device in which the above is realized. The device takes advantage of the fact that the pulsed diode laser acts as a single mode laser in accordance with the selective feedback of its wavelength within the allowable bandwidth of the nonlinear optical medium 31. Since it is certain that the wavelength is within the allowable bandwidth, the magnitude of the frequency augmentation radiation is significantly increased, for example 15 times under some conditions, with respect to the size of the device shown in FIGS. 2 and 3.

The effect of selective feedback is illustrated in FIGS. 5A and 5B. FIG. 5a shows the diode laser spectrum with feedback, and FIG. 5b shows the spectrum emitted by the diode laser when there is no selective wavelength feedback. In these figures, the wavelength λ is constructed on the horizontal axis, and its intensity lies on the vertical axis.

Optional feedback means 37 of FIG. 4 to minimize the effect of undesired feedback due to the reflective action of diode laser radiation on the optical device in devices in which frequency augmentation radiation is used and in which frequency boosting radiation is used. The radiation pulses shown by) have such energy to arrive at the diode laser at the moment when it can have the greatest impact on the diode laser radiation. The intensity condition and delay time condition for which the reflected laser pulse portion LP _r is satisfied can be maintained in relation to the sixth degree. In this figure, pulses LP _i of a plurality of laser pulse series are shown as i = 1, 2, ..., N. The pulse duration of the laser pulse is P and the duration of the series is T. In order for the reflected pulse LP _r to affect the pulse LP _i emitted by the diode laser, the delay time R _t of the reflected pulse must be within a given range. Moreover, the resulting result will be highly dependent on the radiation energy of the reflected pulse with respect to the radiation energy of the pulse establishment.

The delay time range limit is given by

R _t = T

R _t = T-p-Δp

In principle, since the reflected laser pulse LP _{r, 1} does not induce the next laser pulse LP ₂ , but induces one of the second next pulse LP ₃ or the continuous pulses LP _i , Can be generalized as:

R _t = nT, n = 1, 2, ...,

R _t = nT-p-Δp, n = 1, 2, ...

The radiant energy condition can be formulated as follows.

E (P _r )〉 E (LP _i )

The radiation pulse reflected at the instant t _{ε, i} within the establishment time of the i th pulse is injected into the laser.

This allows the reflected pulse to affect the diode laser action at the instant t _{ε, i} so that the radiation energy of the reflected pulse is established at that moment in the laser for the next pulse to be emitted by the diode laser. It must be greater than energy.

Limit of upper limit if R _t = n. If T is satisfied, the trailing edge (falling edge) of the reflected radiation pulse will coincide with the moment when the next pulse to be emitted is fully established, and no longer cause the reflected pulse to have any effect on the pulse to be emitted. If the lower limit R _t = nT-p-Δp is satisfied, then the leading edge (rising edge) of the reflected pulse will coincide with the moment when a new pulse has not yet started so that it is not affected.

The above limitations are not absolute limitations. Depending on the circumstances, some impact may still occur if these limits are exceeded somewhat.

On the other hand, the delay time, i.e. the time required by the radiation pulse LP _i which reverses the diode laser for covering the path from the exit face of the diode laser, is given by 2d / c, where d is the diode laser 12 and Is the distance between the feedback elements 27, and c is the speed of propagation of light in the medium penetrated by the radiation beam. With respect to the two constraints, the following delay conditions are followed.

nT-p-Δp <2d / c <nt

The distance d is given by

d = c / 2 nT-c / 2 epsilon (p + Δp) where epsilon is greater than zero and is a number less than two determined by the energy of the reflected pulse. If the energy is relatively large, the reflected pulse arrives at a moment later than the time at which it is established, so that ε is closer to zero than one. If the energy of the reflected pulse is lower, the pulse arrives at a moment earlier than the time it is established, so that ε is closer to 1 than zero. Ε is inversely proportional to the reflected pulse energy.

Light feedback can be realized by disposing a minimum partial reflecting element 45 having a suitable reflection coefficient at a distance d from the diode laser 12. By placing all other elements that may reflect radiation at a distance that does not meet the general conditions described above for d, the operation of the diode laser and thus the parameters and quality of the laser beam are only the first mentioned elements 45. Determination is made only by the feedback and remains constant through.

However, the position of the wavelength selecting element 47 is not essential as long as the above-described delay time condition is satisfied by the reflecting element 45.

Indeed, the feedback element 37 may be provided in different ways.

The first possible method is shown in FIG. 4, in which the wavelength selecting element 47 is a grating. This grating reflects incident laser radiation to the reflecting element 45, for example a mirror. The radiation reflected by the mirror is reflected by the grating against the diode laser 12. The direction in which the grating reflects radiation is determined by the wavelength of the radiation and the period of the grating, ie the distance between two identical grating strips. The period may be selected by a method such that only radiation having a wavelength within the permissible width of the nonlinear optical medium enters the diode laser.

In an embodiment of the device wherein the diode laser is a KTP (potassium titanyl phosphate) waveguide in which the radiation is transmitted at a wavelength of about 850 nm (red light) and the nonlinear optical medium is a frequency of the diode laser signal doubled. The grating has a period of 1.67 μm, ie 600 strips per mm. The average laser power is about 20 mw with a 1: 3.5 ratio between the emitted pulse width and the pulse repetition frequency, and the power of the double frequency signal (blue light) is about 150 Hz. Temperature stabilization does not need to be performed.

Since the different laser modes are closely located at each other, for example at 0.3 nm, as can be seen from FIG. 5a, the grating has a large dispersing force, so that radiation of different wavelengths is spatially well separated. The dispersion force depends on the number of grating periods covered by the incident radiation beam. When radiation diffracted at two or higher dimensions is used for the feedback, the grating with the desired dispersing force can have a relatively large grating period so that it can be manufactured in a relatively simple manner.

The grating may be manufactured by known methods by mechanical processes (scratching) or lithographic processes. Such a grating may also be obtained by causing two planar radiation waves to interfere on the photolithographic plate and developing and etching the photolithographic plate. Then a laser photogrid is obtained. By adapting to wavefronts, optical properties such as a range of focusing or dispersion or correction can be provided to the laser photo grating.

Many inexpensive replicas can be obtained by using support in which the grid structure is provided as a matrix in the replication process in one of the manners described above.

The grating may also be the planar grating 50 shown in FIG. In this figure, a photoconductive layer 52 is provided on a substrate 51 made of, for example, glass, transparent composite, semiconductor or crystals such as lithium niobate. The layer 52 is composed of a transparent body having a higher refractive index than that of the substrate so that a significant portion of the beam energy coming from the left side remains enclosed in the layer. The layer 52 is provided with a two-dimensional periodic structure of curved extending regions 53 which alternate in the X direction with respect to the extending intermediate region 54. The region 53 may be located at a level higher or lower than the middle region 54 of the layer 52. It is possible that the regions 53 and 54 are located at different refractive indices at the same height. The structures 53, 54 reflect in any direction the radiation connected to the layer 52 according to the wavelength of the radiation shown in FIG. 7 by b ₁ , b ₂ and b ₃ . See, for example, US Pat. No. 4,746,136 for a description of the structure and operation of the planar grating 50. Here, the use of a grating of a multiplexer for an optical waveguide communication system is described.

The planar grating may be used instead of the grating 47 in the device shown in FIG. On the other hand, the device may be simplified when using the reflective characteristics of the grating as shown in FIG. The lens system 35 ′ must allow radiation from the nonlinear element to be focused in the photoconductive layer 52. The planar grating element 50 can be arranged in such a way that the normal to the front 56 extends at a small angle with respect to the main beam of the incoming beam, so that the direction of the reflected beam component b 'with the desired wavelength is It is opposite to the direction of the diode laser beam b. A much simpler embodiment of the device is the grating is integrated into the nonlinear optical medium 31 'as shown in FIG.

In the device according to the invention, a grating with a period of linear extension and a period that changes repeatedly can be used as shown in FIG. In this figure only three grating parts with periods P ₁ , P ₂ and P ₃ are shown for clarity and the differences between the periods are exaggerated. In fact, the grating has a different period with more grating portions, and the difference between the periods becomes smaller. Each of the grating portions located back and forth with each other in the direction of propagation of the radiation beam b will reflect radiation at the characteristic wavelength of the relevant portion in the diode laser. By adjusting a specific value of the laser pulse repetition frequency, the delay time condition can be reliably satisfied by the radiation pulses from the grating portion reflecting radiation in the diode laser and having a wavelength within the allowable bandwidth of the nonlinear optical element.

In devices in which one or more lens systems have been replaced by one or more optical fibers, the grating may be arranged in such optical fibers. An embodiment in this case is shown in FIG. Reference numeral 60 in this figure represents a specific envelope for the optical communication system and incorporates a diode laser 12 and a lens system 62 to illuminate the diode laser output on the entrance face of the optical fiber 71. The wavelength selective optical element 70 may be implemented as described in British Patent Application No. 2,161,648. In this device a portion of the optical fiber 71 is provided in a curved tube of block 73 of glass, for example. The upper end of this optical fiber piece is grounded away from the core 72 provided with the grating 74. The remaining space between the grating and the fiber piece is filled with a liquid having a suitable refractive index. A portion of the radiation propagating through the fiber core is dispersed against the liquid and reaches the grating that deflects the radiation and then enters the fiber core. The period of the grating determines the wavelength of the radiation that interferes with the structurally passed light after interaction with the grating. For a detailed description of the optical device 70, see German Patent Application No. 3254754, which describes a device for stabilizing a diode laser via wavelength-selective feedback.

Optical fibers with an integrated grating may alternatively be obtained by etching the periodic structure within the cladding of the optical fiber.

In the embodiment of FIG. 11, it is arranged between the diode laser of the wavelength selecting element and the nonlinear optical medium 31.

The device 50 of the device shown in FIG. 8 can also be arranged in this position.

12 illustrates an embodiment in which the wavelength-selective fiber component 70 is disposed behind the nonlinear optical element 31 and the optical fiber 75 is disposed between the last mentioned element and the laser.

The grating 74 in the component 70 is preferably a reflective grating that reflects a portion of the radiation passing through the optical fiber 71 and directed to the diode laser. If the grating is placed at a position where the derived delay time condition is applied to the radiation pulse reflected by the grating, the reflector 45 can be removed.

13 shows another embodiment of a radiation source unit 13 with a component 70 with a reflective grating. The embodiment shown in this figure does not have a reflector 45 and an optical fiber 76. This embodiment has the advantage that the frequency increased radiation does not pass through the optical fiber 71 which is optimal for diode laser radiation. Frequency increased radiation can be generated directly from the nonlinear optical medium 31.

This also has the device shown in FIG. In the devices shown in FIGS. 11 and 12, the frequency increased radiation can be coupled to the outside of the device by acting the reflector 45 as a dichroic mirror which allows it to pass but reflects laser radiation. In the apparatus according to FIG. 8, the frequency increased radiation reflects itself but cannot be combined by placing a dichroic mirror 58 between the nonlinear optical medium 31 and the planar grating 50, which passes the laser radiation. . In the apparatus according to FIG. 4, the reflector 45 can act as a dichroic mirror that passes frequency increased radiation. If the reflecting grating 47 is positioned at the position of the reflector 45 to remove the reflector, a dichroic mirror for coupling out the frequency increased radiation is placed between the grating and the nonlinear optical element.

The wavelength selection and the function of reflecting toward the diode laser can be performed with a Fabry-Perot etalon as shown in FIG. The etalons have two partially reflective, planar or curved surfaces including media such as, for example, air or glass. Since multiple reflections occur on the surface, interference is established or broken between the beam portions. The distance W between the surfaces appropriately selects the refractive index of the medium so that radiation having a predetermined wavelength is reflected. This wavelength is of course present within the reception bandwidth of the nonlinear optical medium 31.

Etalons are used for reflection as shown in FIG. 14, but are also used for transmission as shown in FIG. 15, although not further described. Transmission etalons may also be used in the apparatus shown in FIG. In that case, the reflector is placed behind this etalon a distance d. This reflector can be a dichroic mirror that completely passes frequency doubled radiation.

The etalons may also consist of optical fibers having a predetermined wavelength. In order for the reflected radiation pulse to return to the diode laser to determine which wavelength it emits, the energy of this radiation pulse must be greater than the radiation energy produced at the moment of return to the diode laser as described above. The energy of the reflected radiation pulse can be influenced by the reflection coefficient of the reflective element 45 or of the elements 50, 31 ', 80 that perform both wavelength selection and reflective functions.

For example, in the case of a high power diode laser in which an optical record medium is recorded and the exit face has a low reflection coefficient, the reflection coefficient of the feedback element, for example the element 80 of FIGS. 14 and 15, plays an important role. do. In fact, it has been found that diode lasers having a low reflection coefficient at the exit face emit shorter wavelengths than diode lasers having a high reflection coefficient at the exit face due to the large charge medium density results of the laser first mentioned during laser operation. In other words, the reflection coefficient and the wavelength of the exit face of the diode laser can be set due to the reflection coefficient selection of the feedback element 80. When the grating is used as a feedback element, this effect does not occur because the grating fixes the diode laser wavelength. However, if etalon is used as the feedback element, the diode laser wavelength can be fixed by the reflection coefficient of this element.

In practice, the distance d between the diode laser and the feedback element, i.e. the distance to obtain the spectral stability of the diode laser, is relatively long, which is an obstacle in compacting the optical device. For example, for a pulse period p of 1 ns, a distance d of approximately 150 nm is required.

According to another aspect of the invention, the optical paths between the diode laser 12 and the feedback element 27 overlap. As a result, the device has two opposing reflectors between which the radiation beam is reflected several times. However, due to stability, one body 20, for example a glass body, made of an optically transparent material is used, where two opposing surfaces reflect so that the radiation beam entering the body is repeated several times. Tolerances are determined during the manufacture of the body. The folding body is not only made of glass but can also be made of other optically transparent materials with sufficient high refractive index, such as transparent synthetic materials.

There may also be different embodiments of the folding body 120. The first embodiment has a planar-parallel plate 121 with a reflective layer 127 having a high reflection on the first surface 123 and the second surface 125, which are shown in FIG. 21A and located opposite each other. The first surface 123 further has an inlet window 129 and the second surface has an outlet window 131. The radiation beam 15 emitted by the diode laser 12 enters the plane-parallel plate 121 passing through the inlet window 129 via, for example, a collimator lens 132 and the beam is at an acute angle. Because they are incident on the surfaces 123 and 125 they are reflected several times by these surfaces. The beam reaches the wavelength-selective reflecting element 27 arranged behind or in the third window 128 in one of the surfaces. In turn, the beam remains in the diode laser path via window 129, across plate 121 in the reverse direction.

The exit window 131 is used to couple the frequency-increased radiation outside the optics, and is eventually formed like a beam splitting mirror. The window 131 has a layer 134 that has a low reflection for high harmonics generated at high reflections for a fundamental wavelength such as may be the layer 127 on the first surface 123 and the second surface 125. Equipped. In that case, the layers 127 and 134 on the second surface 125 constitute one non-blocking layer.

The wavelength selective feedback element 27 may be a separate component. However, this element is preferably integrated with the planar-parallel plate 121. In other words, it is preferably arranged at the position of the window 128. This device may be a prism, for example. The prism can be formed by placing in or recessing the window. However, the grating 27 as shown in FIG. 21A is preferred because of its relatively high wavelength-resolution.

The grating is integrated into the glass body in different ways. For example, the grating is etched directly in the glass folding body or arranged as a separate component on the folding body. Another possibility is to provide a thin layer of synthetic material on the folding body in which the grating is continuously provided by a replication technique.

Due to the folding, for a radiation beam with a diameter of 3 mm a radiation path having a length of 130 mm, for example in air, has a thickness D of 8 mm and a length L of 13 mm if the glass has a refractive index of 1.8. It can be housed in a glass plate.

Folding selectively affects three dimensions instead of two dimensions. For this purpose, the back-directing element 133 is arranged on the fourth window at the end of the first light path 135 on the glass body as shown in FIG. 22. After the radiation beam passes through a first light path 135 located in a first plane perpendicular to the reflective surface of the glass body, it is first reflected in a direction perpendicular to the plane of the drawing and parallel to the direction of incidence on the element 133. As it is continuously reflected in the direction, the beam passes back to the plane-parallel plate 121 to pass through a second light path 137 located in a second plane parallel to the first plane. The light paths 135, 137 and possible additional light paths folded in this way are superimposed so that the folding body with a shorter length L continuously achieves the light path with the total length d required for spectral stability. Can be used. This embodiment is shown in FIG. 22a is a top view and 22b is a side view.

A very suitable example of the reverse pointing element 133 is a prism having a pole angle of 90 °. The upper rib of this prism is perpendicular to the main beam of the beam. The prism is based on such a method that there is no return loss because the base surface 139 located opposite the upper rib is parallel to the surface 125.

In another possible embodiment the folding body 120 according to the invention has a rectangular or rectangular cross section as shown in FIG. In this embodiment the surfaces 145, 146, 148 and 149 are directed relative to the incident radiation beam in such a way that there is a total internal reflection of the beam on each surface. In the FIG. 23 embodiment this affects twice for each surface before the beam reaches the feedback element 27 arranged inside or behind the window 153.

After reflection by this device, the beam passes through the same radiation path in the reverse direction and leaves the folding body through surface 145 on the way to the diode laser.

Frequency augmented radiation leaves the optical device through surface 149. At the point at which this is realized, a wavelength selective reflecting layer 151 is provided, which has a high reflection on radiation from the diode laser and transmits the frequency amplified reflection. In addition, the prism 147 is mainly provided at this point, and its surface 150 is perpendicular to the main beam of the beam. Similar prisms 143 with vertical surface 152 are also primarily arranged on surface 145 at the position where the diode laser beam is incident on the folding body. Both prisms are made of a material, for example a glass body. Likewise in an embodiment comprising a planar-parallel plate, the feedback element 27 is a prism or etalon which is arranged inside or behind the window 53 in the glass body.

In the same way as shown in FIGS. 22a and 22b for a planar-parallel plate, the folding body of FIG. 23 has a plurality of parallel planes inside the glass body superimposed by the reverse-directing elements to realize folding in three dimensions. It includes a folding light path.

Two embodiments 121 and 122 of the folding body may alternatively be implemented in such a way that the inlet and outlet windows are located on the same surface and coincide. In that case the frequency multiplied radiation is separated from the diode laser radiation by an additional wavelength selection element, for example a wavelength sensitive beam splitter, between the frequency multiplied medium and the folding body, and is coupled to the outside of the diode.

In each of the above embodiments of the folding body with a grating, such as a feedback element, in the form of a planar-parallel plate on the surface 125 or 149 as shown for the grating in FIGS. 21B and 21C for the folding body. The grid can be arranged at an acute angle. In fact, the wavelength resolution depends on the number of lattice period pollings inside the radiation beam and therefore on the diameter of this radiation beam. By inclining the grating relative to the surface of the glass body, the larger surface of the grating is covered by the same radiation beam and consistently higher wavelength resolution can be achieved.

In the glass body of each embodiment, the wavelength to be reflected can be varied so that the body can align in the optical device 1 to rotate about the projecting beam 15, both as a prism and as a feedback element, as a feedback element. have.

Such devices with wavelength selective feedback elements are significantly less sensitive to temperature changes than known devices to supply frequency augmented radiation. The stability of the new device is mainly determined by the temperature dependence of the nonlinear optical medium. If the medium consists of waveguides that are part of KiP, this change is on the order of 0.05 nm / ° C. Various degrees of Celsius change are allowed for nonlinear optical media with a tolerance of 0.3 nm.

A relatively large error with respect to the position of the wavelength selective feedback element is further allowed. If a radiating pulse with a pulse period of 350 psec and a pulse repetition frequency of 900 MHz is used, it corresponds to a pulse period of about 1110 psec and a change of 10 to 20 nm corresponding to a delay time change of a class of 100 psec. Replacement of the feedback element will affect the operation of the device.

Since the required allowable bandwidth is smaller, the temperature dependence of the device increases. Embodiments of devices that utilize this feature have photoelectric feedback in addition to optical feedback. This embodiment is shown in FIG.

A portion of the frequency boosting radiation is detected by the detector 39 and its output signal is referred to as a reference signal in the comparator 41 which supplies a control signal Sc to the temperature control unit 43 of the nonlinear optical medium 31. S _ref ). This is ensured by the feedback element 80 in which the wavelength of the laser radiation is kept constant. The tolerance band is corrected through the photoelectric feedback 39, 41 and 43 in such a way that the laser wavelength is within the tolerance band at a larger temperature change in which the tolerance band of the medium 31 changes.

The left part of FIG. 15, which corresponds to FIG. 14 in etalon as the feedback element, is shown as an example. Combinations of optical and photoelectric feedback of wavelength selection are also possible if the optical feedback is implemented in a grating as shown in FIGS. 4, 8, 9, 11, 12 and 13.

If etalon is used as the feedback element, instead of the nonlinear optical medium, the diode laser 12 can be controlled by varying the current through the temperature of the laser. 17 shows a combination of embodiments connecting the devices of FIGS. 2 and 16 and does not require further explanation. If the feedback element is an etalon, this combination is natural because the wavelength is periodically fixed by the combination of the distance between the diode's output spectrum and the two sides of the etalon. When the grating is used as a feedback element the laser wavelength is fixed and not at least for corrections at large temperature variations so it is not natural to use the feedback of FIG. 17.

However, when a grating is used, for frequency doubling radiation where a diode laser current or temperature is detected, for example to obtain fine tuning between the allowable band of the nonlinear optical medium and the diode laser wavelength within the mode distance of the output spectrum of the diode laser. Can be corrected.

It is possible to arrange a grating on a piezoelectric element to which a signal corresponding to the detected frequency doubling radiation is applied. The fixed wavelength of the grating can thus be corrected by feedback for the grating angle.

A pulsed laser beam can be obtained by operating the diode laser with a periodically varying current. This is a pulse current but for example also a sine current. Diode lasers having such a structure for supplying a pulsed beam are alternatively used. Such lasers, which are usually referred to as self pulsed lasers, are known, for example, from British Patent Application No. 2 221 094.

If the frequency is increased, for example frequency doubled emission (Ru), is used to write the optical recording medium, the radiation is emitted according to the information to be written between the maximum level that occurs in the radiation sensitive layer of the recording medium and the minimum level where this does not occur. It should be possible to switch the strength of the fast.

The present invention offers the possibility of switching the current through the diode laser in a manner other than the known manner. The first possibility is to switch the repetition frequency of the current and to switch the one of the laser pulse between the value when the delay time condition is satisfied and the value when the condition is no longer satisfied. This possibility is shown in FIG. 18.

The device shown in this figure is based on the same theory as that in FIG. 4 and includes the author circuit 90 as an additional component, in which the supply information signal, for example a voice or video signal, is to be recorded. S _i is digitized and coded in a conventional manner. The output signal S _M of the electronic circuit 90 is supplied to a control circuit 91 for a diode laser 12, which control circuit 91 matches one of a series of zeros and a signal S _M. This includes a power supply 92 as well as a sub-circuit for switching the repetition frequency between the two values.

Figure 19 shows the second possibility of switching the intensity of the frequency boost radiation between the maximum and minimum values. The device shown in this figure is based on the same theory as that in FIG. 8 and includes an electronic circuit 95 for converting the signal Si to be written into the voltage between the output terminals 96 and 97, The voltage is switched between two potentials in accordance with the signal S to be written. This voltage is supplied to the electrodes 101 and 102 of the optoelectronic device so that its refractive index is switched between the two values. Thus, the optical path length to be covered by the radiation pulses in the diode laser 12 with the wavelength selective reflector may be switched between a value where the delay time condition is satisfied to produce radiation with an increased frequency and a value other than that. Can be.

A third possibility of switching the intensity of the frequency augmented radiation is to switch the energy of the radiation pulse returning to the diode laser between the first and second values, respectively, greater or less than the energy inherent in the diode laser upon return. For this purpose, the device must include a reflector with a fast corrected reflection coefficient. Such a reflector can be combined with a transmission correctable component to form a reflective component having a fixed reflection coefficient. This component may consist of a liquid crystal element and a polarization analyzer. In FIG. 20 these elements are labeled 110 and 114, respectively. Device according to the drawing is converted to a voltage of between 14 degrees to it based, and includes an electronic circuit, the electronic circuit, the signal S _i are both output devices 106 and 107 are to be written, the voltage information to be recorded Is switched between two potentials. This voltage is supplied between the electrodes 111 and 112 of the liquid crystal cell 110.

The polarization state of the radiation propagating through the cell is switched by means of this voltage and the analyzer switches the two polarization states to two intensity levels.

Instead of the liquid crystal material, other photoelectric birefringent materials can be used instead. The birefringent element and analyzer can be better replaced with a planar interferometer, the planar interferometer with which the refractive index is photoelectrically switched in at least one of the branches.

A fourth possibility of switching the intensity of the frequency augmented radiation is to switch the refractive index of the nonlinear optical medium photoelectrically between two values, ie by the electric field across the nonlinear optical medium. These values are chosen such that the allowable band of the medium associated with one value includes the laser wavelength and the allowable band associated with the other value does not include this wavelength.

All different possibilities regarding switching strength may be used in various embodiments of the device. 18, 19 and 20 illustrate different embodiments of the device, respectively.

As already mentioned, the choice of nonlinear optical media depends on the desired tolerance band. The medium may also have a different shape. The medium may be a waveguide 31 (2nd, 3rd, 8th to 15th, 17th, 18th and 20th) in which the radiation inducing layer comprises a nonlinear optical material. Suitable materials for this are, for example, KTP, LiNbO ₃ or LiTaO ₃ . The medium may also be a nonlinear optical crystal 31a (FIGS. 4, 16 and 19). Suitable materials for this are, for example, KNbO ₃ or KLiNbO ₃ . Compared to waveguides, the crystal has the advantage that the bond is mechanically more stable. However, despite the loss of coupling phase, the presence of the waveguide allows a high conversion efficiency of diode laser radiation to be achieved.

The description of the present invention is based on an optical device for reading and / or writing an optical disc, in which the frequency with respect to the radiation of the diode laser is more preferably doubled. The temperature independent and feedback unresponsive halving of the wavelengths realized according to the invention is also advantageous for other devices such as, for example, printers and scanners or projection devices for the production of integrated circuits, liquid crystal display panels, etc. by photolithography. To provide.

Half-wavelength, for example, the printer offers the possibility of using other radiation sensitive materials with higher sensitivity than the materials used so far, requiring lower radiant energy for printing, or better printing results with the same radiant energy. Can be achieved.

The present invention is not limited to frequency doubling or wavelength halving, but may be used to increase the radiation at the radiation source to different sizes, and may also be used to obtain radiation at a given frequency by mixing two radiation sources having different frequencies.

Claims (36)

Frequency increased by electromagnetic radiation, including a diode laser that supplies electromagnetic radiation to increase frequency, and a non-linear optical medium having an acceptable bandwidth and ensuring frequency increase. In an optical device, the diode laser is a pulsed diode laser, and feedback means is provided to set the radiation supplied by the diode laser at a wavelength within the allowable bandwidth of the nonlinear optical medium. Optical devices. 2. The active control system of claim 1, wherein the feedback means comprises a control unit controlled by a detector sensitive to frequency boost radiation and an output signal of the detector affecting at least one parameter determining the amount of frequency boost radiation. Optical device, characterized in that formed by. The optical device of claim 2 wherein the parameter is a current flowing through the diode laser. The optical device of claim 2 wherein the parameter is a temperature of the diode laser. An optical apparatus according to claim 1, wherein said feedback means is formed by optical wavelength selective feedback means. 6. The apparatus of claim 5, wherein the wavelength selective feedback means comprises at least partially reflecting feedback elements arranged at a distance d from the diode laser, the distance d being a condition of: d = c / 2 nT-c / 2 ε (p + Δp) where p is the pulse duration of the emitted laser pulse, T is the pulse period, n = integer, and c is the medium through which the radiation beam passes. The speed of light, p, is the formation time of the pulse LP in the diode laser, and ε is a real number satisfying 0 <ε <1 and increases or decreases within these limits to the increasing or decreasing energy E (Pr) of the radiation pulse reflected by the feedback element. Or decreases, and when this radiation pulse enters the diode laser, E (Pr)〉 E (LPi) Is satisfied, wherein E (LPi) is the radiant energy formed in the diode laser at the appropriate moment. 7. Optical device according to claim 5 or 6, wherein the wavelength selective feedback means comprises a grating. 8. The optical device of claim 7, wherein the grating is a holographic grating. 8. The optical device of claim 7, wherein the grating is a planar grating present in an optical fiber subsequent to the medium. 8. The optical device of claim 7, wherein the grating is a planar grating arranged on the medium. 7. An optical apparatus according to claim 6, wherein said wavelength selective feedback means is formed from one element that partially reflects and selects a wavelength. 12. The optical device of claim 11 wherein the device is etalon. 12. The optical device of claim 11 wherein the device is a partially reflective grating. 13. The method according to any one of claims 1, 5, 6, 11 or 12, wherein a folding means for folding the radiation path is arranged between the diode laser and the feedback means. An optical device characterized by the above-mentioned. 15. The system of claim 14, wherein the folding means comprises a folding body made of an optically transparent material having at least two reflective surfaces and equipped with an entrance window and an exit window. And a third window for transmitting diode laser radiation to or from the feedback means on one of the reflective surfaces. 15. The optical device according to claim 14, wherein said feedback means is integrated in said third window. The optical device according to claim 15, wherein each reflecting surface is provided with a layer having a high reflection coefficient. 18. The optical device of claim 17, wherein the high reflection coefficient layer has a high reflection coefficient for radiation supplied by a diode laser and a low reflection coefficient for frequency augmented radiation. 16. The optical device according to claim 15, wherein the folding body is a planar parallel plate in which the first reflecting surface and the second reflecting surface are opposed in parallel to each other. 16. The system of claim 15, wherein the folding body is present in a medium having a refractive index less than the refractive index of the body material and the folding body has at least two sides with a totally internal reflected radiation incident thereon and the radiation being foldable. And at least once reflected by each one of said two faces when traversing a coplanar radiation passage in the body. 16. The optical device of claim 15, wherein an optical prism is arranged above the inlet and outlet windows, and the face of the prism in which the radiation beam enters the prism and leaves the prism lies transverse to the main beam of the beam. 16. The apparatus of claim 15, wherein one of the reflective surfaces is provided with a fourth window in which reverse pointing elements are arranged, and radiation by the reverse pointing elements traverses a first radiation passage extending through the plurality of reflections to the reflective surface. And traverse at least a second radiation passage which is captured in a first plane within the folding body and reflected parallel to itself and extends through the plurality of reflections in the plane parallel to the first plane to the reflective surface. 16. The optical device according to claim 15, wherein the feedback means comprises a grating, the grating extending at different small angles from 0 ° to the third window. 17. The optical device of claim 16, wherein the folding body is rotatably arranged over a small angle with respect to the radiation beam supplied by the diode laser, in order to vary the wavelength of radiation reflected towards the diode laser. 14. An optical wavelength selective feedback means according to any one of claims 5, 6, 11, 12 or 13, wherein the optical wavelength selective feedback means is at least one for determining a detector sensitive to frequency amplified radiation and the amount of frequency amplified radiation. An optical device supplemented by an active control system comprising a control unit regulated by an output signal of the detector that affects a parameter. 27. The optical device according to claim 25, wherein said parameter is a current flowing through a diode laser and said control unit controls said current. 27. The optical device according to claim 25, wherein the parameter is a diode laser temperature and the control unit controls the temperature. 27. The optical device of claim 25, wherein the parameter is a temperature of a nonlinear optical medium and the control unit controls the temperature of the medium. 27. The optical device of claim 25, wherein the parameter is a refractive index of a nonlinear optical medium and the control unit controls the magnitude of the electric field across the medium. 14. A diode laser according to any one of claims 1, 2, 3, 4, 5, 6, 11, 12, or 13, wherein the diode laser is a magnetic pulse diode. An optical device, characterized in that a laser. The nonlinear optical medium of claim 1, 2, 3, 4, 5, 6, 11, 12, or 13, wherein the nonlinear optical medium is a nonlinear optical medium. And a waveguide formed from the material. The nonlinear optical medium of claim 1, 2, 3, 4, 5, 6, 11, 12, or 13, wherein the nonlinear optical medium is a nonlinear optical medium. Optical device comprising a crystal (crystal). 32. The optical device of claim 31 wherein the waveguide is formed from KTP and is formed from one of the materials LiNbO ₃ or LiTaO ₃ . 33. The optical device of claim 32, wherein the nonlinear optical crystal is KNbO ₃ or KLiNbO ₃ . An information plane having a radiation source unit, an optical system for focusing radiation supplied by the radiation source unit at a scanning point in the information plane, and a radiation sensitive detection system for converting radiation from the information plane into an electrical signal 13. An apparatus for optically scanning a light source, wherein the radiation source unit comprises at least one of claims 1, 2, 3, 4, 5, 6, 11, 12, or 13. Apparatus for optically scanning onto an information plane, characterized by an optical device as claimed in claim 1. An apparatus for recording information in a radiation sensitive layer on a medium comprising a radiation source and an intensity switch for switching the beam intensity supplied by a radiation source in accordance with the recorded information. A device according to any one of claims 1, 2, 3, 4, 5, 6, 11, 12, or 13, wherein the strength switch is: Means for setting one of the following parameters: the repetition frequency of the diode laser pulse, the optical path length of the radiation pulse returning to the diode laser, the energy of the radiation pulse returning to the diode laser, and the allowable band of the nonlinear optical medium. An information recording apparatus, characterized in that consisting of.