IL105863A - Frequency multiplier - Google Patents

Description

105,863/2 Ί3ΐ κ/η ΠΙΤΊΠ ' osn IMPROVED FREQUENCY MULTIPLIER STATE OF ISRAEL-ATOMIC ENERGY COMMISSION Inventors: David Eger;. Moshe Oron Moti Katz; Avigdor Zussman. inm nv TAX TIT itPN'snnn inpi T ΙΠΑΊΚ , γι 'urn 16622s03 IMPROVED FREQUENCY MULTIPLIER FIELD OF THE INVENTION The present invention relates to laser/frequency conversion in general and, more particularly, to frequency multipliers for use with diode lasers.

BACKGROUND OF THE INVENTION There has been much recent effort directed toward the development of non-linear optical devices for laser frequency conversion. Particularly, attempts have been made to construct compact laser devices, using diode lasers and frequency-converting waveguides. In principle, a fundamental frequency beam is emitted by a laser diode and is then guided through an optical non-linear wave guide which doubles the fundamental frequency and, thereby, generates a converted-frequency beam. Normally, the fundamental frequency is in the infrared range and the second-harmonic frequency is in the visible range, thereby producing a useful output of visible laser light. Hopefully, such f equency-doubling will be utilized to construct compact laser sources which may be useful, for example, in medical and metrological instruments and for optical recording.

In order to obtain reasonable results from frequency multipliers using optical non-linear devices, the phases of the fundamental wave and the second-harmonic wave must be perfectly matched. Somech et. al. in Applied Physics Lett. 21, 1972, suggest using quasi-phase matching (QPM) which is based on periodically changing the refractive index and the non-linear coefficient of the wave-guide. For efficient multiplication of the fundamental wave, using QPM, the wavelength of the fundamental wave must be within a narrow range, of no more than 1 or 2 A, centered at a wavelength which satisfies the specific QPM condition. Consequently, it is extremely difficult to match between the fundamental frequency generated by the diode laser and the frequency which the QPM wave-guide is adapted to double. Actual QPM wave-guide frequency converters have been constructed using materials such as, for example, LiTa03 and LiNb03.

U.S. Patent 5,028,107 to Bierlein et. al. describes a frequency converter which includes periodically segmented wave- 16622s03 guides formed of a KTiOPC^ (KTP) crystal or the like. To form the wave guide pattern, first, a masking material is applied to a smoothed surface of the KTP crystal, providing a pattern of aligned regions along the surface. Then, a preselected amount of molten salt containing cations such as Rb+ or the like is applied to the smoothed, partially masked, surface of the KTP crystal for a preselected time period and at a preselected temperature, resulting in a preselected amount of cation replacement at the unmasked regions of the surface. The cation replacement at the unmasked regions changes the index of refraction of a portion of the crystal under the unmasked regions. Finally, the masking material is removed and the edges of the crystal are polished to provide clean input and output faces. ■ In "Compact Blue-Green Laser", presented at a Topical Meeting, on Feb. 2-4, 1993, and published in OSA Technical Digest Series, Vol. 22, PP. 301, 480 and 489, there is described a method of coupling a diode laser to a QPM frequency converter using an external grating. According to this method, infrared light emanating from the output face of a wave guide is reflected by an external grating back through the wave-guide and into the laser diode. Due to this reflected light, the frequency generated by the laser is stabilized to a preselected level, which is controlled by the angle of the grating. Unfortunately, such a system is not useful for most applications of compact visible-light lasers because it is complex and expensive and, especially, because it is extremely sensitive to changes in ambient conditions.

U.S. Patent 5,185,752 to Welch describes an arrangement for coupling a diode laser to frequency doubling wave-guide. This Patent suggests the use of a distributed reflective grating which is formed at the input face of the wave-guide. The grating reflects infrared radiation of a preselected fundamental frequency, determined by the spacing of the grating, back into the laser cavity, thereby stabilizing the output of the diode laser at the preselected frequency. The stabilized fundamental frequency is then doubled by a nonlinear wave-guide, downstream of the grating, in the manner described above. 16622s03 The proposed frequency-doubler of Patent '752 poses a number of problems. One problem is that the reflected portion of the fundamental wave is not utilized by the frequency doubler. It should be appreciated that the infrared output power of the diode laser is reduced, due to loss of the reflected radiation, by a factor of (1-R)2, wherein R is the reflectivity of the grating. It should be noted that, for practical purposes, reflectivity R is preferably equal to at least 0.2 and, therefore, the conversion efficiency of such a device will be reduced to no more than 64% of the original efficiency of the doubler wave-guide.

Another problem of Patent '752 relates to frequency matching. The Patent does not provide appropriate means for fixing the wavelength of the reflected wave with the accuracy required for frequency doubling. The method disclosed by Patent '752 for creating a first or second order grating on a substrate includes holographical exposure of the surface of the substrate through a selective mask, followed by etching of the exposed portions of the surface at the input face of the wave-guide. It should be appreciated that such a method has not, to date, provided the accurate wavelength matching, in the order of 1 - 2 A per 10,000 A, required by frequency doublers.

Roelofs et. al. in "Compact Blue-Green Lasers" , presented at a Topical Meeting, on Feb. 2-4, 1993, aftd published in OSA Technical Digest, Series 2, P. 485, describe a compact frequency doubler, including a diode laser and a periodically segmented wave-guide. The periodic segmented pattern is designed to reflect part of the input radiation and to convert the frequency of some of the unreflected input radiation. The reflection from the wave-guide is used for stabilizing the frequency generated by the laser diode. This arrangement requires the use of a wave-guide which meets both the QPM, i.e. phase matching, and Bragg, i.e. frequency matching, conditions described above. Due to the Bragg condition requirement, a considerable fraction of the fundamental wave is reflected and is not used by the frequency doubler. The energy carried by the wave decays exponentially as a function of distance traveled,within the boundaries of the wave-guide and, therefore, the power actually used for doubling is thus reduced. 16622s03 ¾ The requirement that both the Bragg?rand QPM conditions should be met simultaneously imposes a substantial limitation on the choice of a fundamental frequency. Furthermore, after such a frequency has been chosen, there are considerable technical limitations to the actual construction of a wave-guide adapted for the fundamental frequency. Firstly, the existing mechanics of segmented wave-guide production methods limits one to a set of discrete dimensions for the sections of the wave-guide. For example at 0.1 μπι addressing, which is typically the case, all the sections of the wave-guide must be integer multiples of 0.1 μπι. Secondly, existing techniques have inherent inaccuracies which result in discrepancies between prescribed, design, values and actual, produced, values of the period length. Thus, according to existing techniques, it is impossible to construct an actual wave-guide which is adapted for a prescribed frequency and meets both of the above mentioned conditions.

Considering the above, Roelofs et. al. proposed an improvement to the apparatus described in the preceding paragraphs, including the use superperiod structures in the construction of the wave guide. The use of superstructures, aided by temperature tuning, improves the ability to adjust the frequency reflected by the wave-guide, but it does not remove the limitation imposed on the choice of frequency.

Another disadvantage of the apparatus proposed by Roelofs et. al., for which their disclosure suggests no solution, is the reflection of the second harmonic frequency, back towards the diode laser. It is appreciated that, in principle, when the QPM and Bragg reflection conditions are both met, simultaneously, for the fundamental frequency, the Bragg condition is also met for the second harmonic frequency. Thus, a substantial amount of second harmonic radiation is reflected back towards the laser diode, thereby reducing the frequency doubling efficiency of the device . 16622s03 SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide an improved frequency-converted laser apparatus. In accordance with one, preferred, aspect of the present invention there is thus provided a frequency-converted laser apparatus including a primary laser oscillator, operative to emit primary radiation, and a Bragg reflector. The Bragg reflector is operative to reflect part of the primary radiation, within a limited frequency range centered at a preselected fundamental frequency, back along an optical path into the primary oscillator. The apparatus also includes a frequency-converter, located on the optical path between the primary laser oscillator and the Bragg reflector, operative to convert part of the radiation at the fundamental frequency into frequency-converted secondary radiation.

This arrangement creates an external laser cavity, including the primary laser oscillator and the frequency converter, which resonates at the preselected fundamental frequency. In a preferred embodiment of the invention, the primary laser oscillator includes a reflective back facet and, thereby, the external laser cavity extends from the reflective back facet to the Bragg reflector.

In accordance with a preferred embodiment of the present invention, the frequency converter and the Bragg reflector are both formed on crystalline material, preferably a KTP crystal. More preferably, the frequency converter and the Bragg reflector are formed on the same crystal.

Further in accordance with a preferred embodiment of the invention, the fundamental frequency is in the infrared range and, more preferably, the primary oscillator is a diode laser emitting infrared radiation. The frequency of the secondary radiation is preferably higher than the fundamental frequency and, more preferably, it is in the visiblegrange. In a particularly preferred embodiment of the invention, "the frequency of the secondary radiation is a second harmonic frequency equal to twice the fundamental frequency.

In a preferred embodiment of the present invention, the frequency converter includes a plurality of periodic segments 16622s03 including sections of a first kind, having a first index of refraction, and sections of a second kind, having a second index of refraction. Further, in a preferred embodiment, the sections of the first kind have an electric polarity opposite the electric polarity of the sections of the second kind. In a particularly preferred embodiment of the invention, each periodic segment is a superperiod Lk including k sub-periods and each sub-period includes one section of the first kind followed by one section of the second kind. $ Similarly, the Bragg reflector preferably includes a plurality of periodic reflecting segments including reflecting sections of a first kind, having a first index of refraction, and reflecting sections of a second kind, having a second index of refraction. In a particularly preferred embodiment of the invention, each periodic reflecting segment is a superperiod fij including j reflecting sub-periods and each reflecting sub-period includes one reflecting section of the first kind followed by one reflecting section of the second kind.

The frequency converter preferably includes an input face for receiving the primary radiation. According to this embodiment of the invention, at least one lens is located between the primary oscillator and the frequency converter for focusing the primary radiation onto the input face of thev.frequency converter.

In accordance with another, preferred, aspect of the present invention, there is provided an integrated wave-guide unit including a frequency converting portion, adapted to receive primary radiation and, upon receipt of the primary radiation, to convert part of the primary radiation into frequency-converted secondary radiation, and a Bragg reflector portion, adapted to receive radiation from the frequency-converting portion and to reflect part of the primary radiation, within a limited frequency range centered at a preselected fundamental frequency, back through the frequency converting portion.

In a preferred embodiment of the invention, the frequency converting portion and the Bragg reflector portion are both formed on a single chip of crystalline material. Preferably, the single chip of crystalline material is a chip of KTP crystal. 16622s03 The integrated unit is preferably adapted to receive primary radiation in the infrared range which is, more preferably, generated by a diode laser. The frequency-converted secondary radiation is preferably in the visible range. In a preferred embodiment, the frequency of the secondary radiation is higher than the fundamental frequency and, more preferably, the frequency of the secondary radiation is a second harmonic frequency equal to twice the fundamental frequency.

In a accordance with a preferred embodiment of the invention, the integrated unit includes a plurality of periodic segments including sections of a first kind, having a first index of refraction, and sections of a second kind, having a second index of refraction. The first kind of sections have an electric polarity opposite the electric polarity of the second kind of sections. In a particularly preferred embodiment, each periodic segment is a superperiod Lk including k sub-periods wherein each sub-period includes one section of the first kind followed by one section of the second kind.

Similarly, the Bragg reflector portion preferably includes a plurality of periodic reflecting segments including reflecting sections of a first kind, having a first index of refraction, and reflecting sections of a second kind, having a second index of refraction. Preferably, each periodic reflecting segment is a superperiod fij including j reflecting sub-periods, wherein each reflecting sub-period includes one reflecting section of the first kind followed by one reflecting section of the second kind.

According to another, preferred, aspect of the present invention, there is provided a frequency-converted laser apparatus including a primary laser oscillator having a reflective back facet and a front facet, operative to emit primary radiation through the front facet, and an integrated wave-guide unit as described in the previous paragraphs. The integrated wave-guide unit preferably includes an input face, optically associated with the front facet of the primary oscillator, for receiving the primary radiation. According to this aspec-t of the invention, an external laser cavity is formed between the reflective back facet of primary oscillator and the Bragg reflector portion of the 16622s03 integrated wave-guide. The external laser cavity resonates at the fundamental frequency. The apparatus preferably includes at least one lens, located between the front facet of the primary oscillator and the input face of the integrated wave-guide unit, for focusing the primary radiation onto the input face of the integrated wave-guide. 16622s03 BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood from the detailed description of the preferred embodiments of the invention taken in conjunction with the following drawings, of which: Fig. 1 is a schematic illustration of a frequency-converted laser apparatus, constructed in accordance with a preferred embodiment of the present invention; Fig 2A is a schematic illustration of an integrated waveguide unit useful in the operation of the frequency-converted laser apparatus of Fig. 1, constructed in accordance with one, preferred, embodiment of the invention; and Fig. 2B is a schematic illustration of an integrated waveguide unit useful in the operation of the frequency-converted laser apparatus of Fig. 1, constructed in accordance with another, preferred, embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Reference is now made to Fig. 1, which illustrates a frequency-converted laser apparatus in accordance with a preferred embodiment of the present invention. A diode laser 10, including a back facet 12 and a front facet 14, emits primary radiation, preferably in the infrared range of 800nm - lOOOnm. In accordance with a preferred embodiment of the present invention, back facet 12 is highly reflective while the reflectivity of front facet 14 is relatively low.

The radiation emitted by diode 10 is focused, preferably by a system of converging lenses such as lenses 16 and 18 shown in Fig. 1, onto the input face 20 of an integrated unit 36. Unit 36 is preferably integrally formed on a single substrate, such as a crystal, as described in more detail below with reference to Figs. 2A and 2B. In a preferred embodiment of the invention, the emitted radiation is focused only onto a small portion of face 20 near the upper surface 40 of integrated unit 36. Therefore, the input radiation is received and guided only through the uppermost layer of unit 36, hereinafter referred to as integrated waveguide 21.

According to the present invention, the radiation entering integrated wave-guide 21 through input face 20 is led, first, 16622s03 through a frequency converting portion 22 and, then, through a distributed Bragg reflector portion 24. Frequency converting portion 22 converts part of the primary radiation to radiation at a higher frequency, preferably in the visible range, hereinafter referred to as the converted-frequency radiation. According to one, preferred, embodiment of the invention, the converted frequency is equal to exactly twice the fundamental frequency and, in such case, the converted-frequency will be referred to as second-harmonic frequency.

In a preferred embodiment, the regenerated converted-frequency radiation is not influenced by Bragg reflector portion 24 and, therefore, it freely exits wave-guide 21 through a transmissive output face 26. The unconverted part of the primary radiation, on the other hand, is reflected by Bragg reflector 24, through portion 22, back into diode laser 10. Preferred embodiments of integrated unit 36, including preferred structures of integrated wave-guide 21, will be described in detail below with reference to Figs. 2A and 2B.

It should be appreciated that when the "arrangement described above is used, an external cavity is formed between back facet 12 of diode 10 and distributed Bragg reflector 24. In a preferred embodiment, distributed Bragg reflector 24 is designed to be highly reflective in response to a preselected frequency nfQn, hereinafter referred to as the fundamental frequency. Preferably, fundamental frequency "fo" Is chosen such that a resonant condition, i.e. a QPM condition for frequency conversion, is met. The external cavity resonates at the chosen fundamental frequency "f0" and, if frequency "fg" is properly adapted to the frequency input requirement of frequency converter 22, a converted, stable, output in the visible range is provided through output face 26.

It is appreciated that the radiation exiting wave-guide 21 through output face 26 mainly includes frequency-converted secondary radiation. However, some residual infrared radiation is also included in the output of wave-guide 21. The output radiation is preferably collimated by a lens 28 and, then, filtered by an infrared filter 30 which filters out the above mentioned residual infrared radiation. 16622s03 It should be appreciated that the efficiency of the proposed apparatus can be improved by coating the inner surface 23 of input facet 20 with a dichroic reflective layer. Such a layer reflects light in the converted frequency range and, therefore, prevents loss of secondary radiation through input facet 20. More efficient results are achieved when thermal control means 34, such as a controlled thermoelectric element or any other suitable thermal control means, is provided to control the temperature of integrated wave-guide 21. By controlling the temperature of waveguide 21, perfect matching between the fundamental frequency and the converted frequency can be maintained.

It should be appreciated that, in contrast to the apparatus suggested in U.S. Patent 5,185,752, which has been described above, the radiation emitted by diode 10 reaches distributed Bragg reflector (DBR) 24 only after passing through frequency converter 22. Due to this arrangement, frequency converting portion 22 is included in the external cavity which is formed between facet 12 and Bragg reflector 24, as described above. Consequently, the energy of unconverted radiation reflected by Bragg reflector 24 is not lost but, rather, it is re-utilized by the resonating external cavity of the present invention. Thus, the power conversion efficiency (i.e. the fraction of primary radiation power converted into secondary emission at the converted frequency) of the present apparatus is considerably higher than that of prior art converters not employing such an external cavity.

It should be appreciated that by placing DBR 24 after frequency converter 22, as suggested by the present invention, the conversion efficiency can be increased by a factor of up to 1+R2, wherein "R", typically between 0.2 and 1.0, is the reflectivity of DBR 24.

It is appreciated that, for best results, the temperature in wave-guide layer 21 must be kept substantially constant temperature in order to maintain the quasi phase matching (QPM) condition between the fundamental wave and the frequency-converted harmonic wave. Therefore, integrated unit 36 is preferably mounted on a heat-dissipative base element 38. Base element 38, which 16622S03 is preferably thermally associated with thermal control means 34, acts as a heat sink for heat produced in wave-guide layer 21. For high power lasers, wherein heat is rapidly produced in wave-guide 21, more efficient heat dissipation is required. In such cases, unit 36 is preferably mounted on base element 38 using upper surface 40, i.e. the surface under which wave-guide layer 21 is formed, contrary to the arrangement shown in Fig. 1.

Reference is now made to Fig. 2A, which illustrates integrated unit 36 in more detail. Unit 36 is preferably made of a single substrate, such as a KTP (i.e. KTiOP04) crystal or any other suitable substrate known in the art. Integrated wave-guide 21, formed in the uppermost layer of unit 36 as described above, is preferably periodically segmented as explained in detail below. As described above, with reference to Fig. 1, wave-guide 21 includes a frequency converting portion 22 (also referred to, herein, as secondary radiation generator) and a distributed Bragg reflector ( DBR ) portion 24.

As can be seen in Fig. 2A, converter portion 22 is divided into segments of equal length "L". Each segment is subdivided into first and second sections and I^-, respectively. In a preferred embodiment of the invention, the refractive index of the L±+ sections is higher than that of the Lj_- sections. Furthermore, the crystal polarity of the sections is opposite that of the L^- sections. DBR portion 22 is divided into segments Ω± of equal length Ω, and each segment is sub-divided into first and second sections Ω^+ and ΩΑ-, respectively. In a preferred embodiment of the invention, the refractive index of the Ω + sections is higher than that of the Ω1- sections, as in the segments, but the crystal polarities of the two kinds of sections are not necessarily reversed.

In accordance with a preferred embodiment of the invention, the reflector portion 24, as described above, meets the Bragg reflection condition, for a frequency "fo" when: (1) β(ί0) = τιΝ/Ω wherein β is the propagation constant for the frequency fQ and N, an integer, is the reflection order.

Frequency converter 22, as described above, meets the QP 16622s03 condition, for doubling of a fundamental frequency "f", when: (2) (2f)-2p(f) = 2nM/L wherein , which is preferably equal to 1, is the QPM order.

It should be appreciated that, Since L and Ω are independently selected parameters, both the Bragg and QPM conditions can be complied with, simultaneously, for any wavelength in the transmission range of wave-guide 21. However, it is appreciated that, for efficient frequency doubling, the following must hold: (3) 6f = |f-f0| < 6 wherein €, typically €/f»10"^, is the line-width of a characteristic doubling curve.

As mentioned above, thermal tuning can improve frequency matching and, therefore, when thermal tuning is used, the burden imposed by equation (3) can be reduced to: (4) 6f < Γ wherein Γ is the change in frequency induced by changing the temperature of the wave-guide within a given controlled range. Typically, r/f*5«10~^. Thus, for an efficient apparatus, periods L and Ω must be properly chosen so that equation (4) holds.

It is appreciated that existing wave-guide fabrication methods, such as using an electron beam mask, feature relatively low resolution in selecting periods "L" and "Ω". Therefore, L and Ω must be selected from discrete values, separated by steps of at least 0.1 μιη. This dramatically reduces the leeway in choosing laser frequencies for which equation (4) holds. The present inventors have, therefore, developed a method of producing improved integrated wave-guides which overcome the above mentioned problem. A preferred embodiment of such an improved wave-guide and a preferred method for producing the improved wave-guide are described below.

Reference is now made to Fig. 2B, which illustrates an alternative, preferred, embodiment of integrated unit 36. As in the embodiment of Fig. 2A, an integrated wave-guide layer 51 includes a frequency doubling portion 52 and a DBR portion 54 which are both periodically segmented. But, unlike the embodiment of Fig. 2A, each periodic segment in the embodiment of Fig. 2B includes more than two sub-sections. Portion 52 is divided 16622s03 into superperiods of length Lk, wherein each segment is subdivided into k sub-periods 41. Portion 54 is divided into superperiods of length Oj , wherein each segment flj is sub-divided into j sub-periods 45. Fig. 2B illustrates one example of such an arrangement, wherein k=3 and j=5, but it should be appreciated that any other suitable values of k and j may also be used, in accordance with specific requirements.

Each of the k sub-periods 41, in each superperiod Lk, includes a first section 42 followed by a second section 44. Sections 42 have properties (i.e. refractive index and electric polarity) similar to those of sections L^+, described above with reference to Fig. 2A, and sections 44 have properties similar to those of sections L^- of Fig. 2A. Similarly, each of the j sub-periods, in each superperiod nk, includes a first section 46 followed by a second section 48. Sections 46 have properties (i.e. refractive index and electric polarity) similar to those of sections Ω^+, described above with reference to Fig. 2A, and sections 48 have properties similar to those of sections Ω1- of Fig. 2A. It can be seen that for k=j=l, portions 52 and 54 of Fig. 2B are identical to portions 22 and 24, respectively, of Fig. 2A.

When the super period construction of Fig. 2B is used, the Bragg condition should be restated as follows: and the QPM condition is restated as follows: (6) p(2f)-2 (f) = 2nk/Lk It should be appreciated that, when such superstructures are used, QPM frequency "f" is controlled by the average length of sub-periods 41, i.e. L^/k. For a given resolution in selecting the length of sub-periods 41, typically 0.1 μπι as described above, the effective resolution in selecting the average sub-period L^/k is improved by a factor of k and, typically, equals 0.1/k μπι. Thus, the limited resolution in selecting frequency "f" can be improved considerably. A similar improvement of resolution can be achieved in selecting Bragg frequency "fo"' which is controlled by the average length of sub-periods 45, i.e.

The average sub-period O j may thus be selected with a 16622s03 resolution of 0.1/j μπι.

It is appreciated that due to additional, random, variance in the reproducibility of integrated wave-guide 51, the frequency difference 6f (6f=|f-fg|) may fluctuate within a frequency range μ which is larger than Γ, as defined in equation (4) above. However, the present inventors have developed a production procedure which overcomes this problem. According to the new procedure, a series of N (Ν»2μ/Γ) successive integrated wave-guides, such as integrated wave-guide 51, are formed" on single chip of a suitable substrate (preferred substrates are disclosed in the examples given below). The wave-guides in the series are prearranged to -have gradually increasing QPM frequencies "f", i.e. each integrated wave-guide in the series is designed to have a target QPM frequency "f" higher than that of the preceding waveguide and lower than that of the subsequent wave-guide. The design QPM target frequencies are typically separated by intervals of α»Γ/2. In contrast, the Bragg frequencies " f0" of the different wave-guides in the series are aimed to have equal target values.

The different wave-guides in the series thus formed are then tested for compliance with equations (3) and (4). It should be appreciated that some of resultant integrated wave-guides are bound to comply with the condition set forth by equation (4) or, even, with the more rigid condition of equation (3). In accordance with a preferred embodiment of the invention, any of the resultant wave-guides complying, in fact, with either of the above mentioned conditions may be used in frequency-converted laser apparatus as described above. The remaining integrated units, failing to comply with both of conditions (3) and (4), are not used and may be disregarded. 1 The substrate bearing the series of integrated wave-guides may be cut at appropriate locations, between adjacent wave-guides 51, thus forming a series of separate integrated units 36. In a preferred embodiment of the invention, any of the resultant integrated units complying, in fact, with the above mentioned conditions are used in frequency-converted laser apparatus as described above. The remaining integrated units, failing to 16622s03 comply with either of conditions (3) and (4), may be discarded.

Alternatively, the series of integrated wave-guides can be designed to have gradually increasing DBR frequencies "fn" and equal QPM frequencies "f". Again, some of the resultant integrated wave-guides are expected to comply with conditions set by equations ( 3 ) or ( 4 ) .

Specific examples of actual integrated wave-guide units, as well as methods of producing such units, are presented in the following paragraphs. These examples are provided for explanatory purposes only and should not, in any way, be understood as limiting the scope of the present invention. Other designs, substrates and fabrication methods, some of which may yield equal, or even better, results when used with the present invention, are also within the scope of the present invention.

A series of integrated wave-guide units was formed on a single chip of a KTP, i.e. KTiOPO^ All of the Bragg reflector portions in the series were designed to reflect a frequency "fg" corresponding to a wavelength of 853 nm. The series included nine integrated wave-guides having nine different frequency doubling portions. Each of the nine doubler portions was constructed in accordance with a superstructure of periodic segments, including 12 sub-periods, such as the Lks of Fig.2B (wherein k=12). The difference in super-period length (Lk in Fig. 2B) between successive doubler portions was 0.1 pm. The shortest super-period, which was 47.8 pm long, included ten sub-periods 41 of 4 pm and two sub-periods 41 of 3.9 pm. Each sub-period 41 included two adjacent sections such as sections 42 and 44 of Fig. 2B. The difference of 0.1 pm, between the super-period lengths Lk of successive doubler portions 52, was obtained by a modification of 0.1 m in one of their respective sub-periods 41.

Each of the nine DBR portions was constructed in accordance with a periodic structure of segments, such as the ±s of Fig. 2A or the QjS of Fig. 2B (wherein j=l). All of the DBR portions were designed to have the same period, 3.7 pm, including a 2.3 m section, such as Ω±+ in Fig. 2A, and a 1.4 pm section, such as section Ω^- in Fig. 2A.

The series of; wave-guides according to the above design was 16622s03 produced using a production method, explained briefly below, similar to the method described in U.S. patent 5,028,107. A Z-cut KTP crystalline chip was coated with a layer of photoresist. The photoresist coat was selectively exposed to light, through a patterned mask made by an electron beam, in accordance with the desired pattern of the series of wave-guides described above. The exposed layer was then developed in order to remove the exposed portions, thereby creating a patterned layer of photoresist. The photoresist layer was then overcoated with a thin layer of Ti which was, then, selectively removed by a lift-off technique, as is well known in the art. Thus, a patterned layer of Ti was created on the surface of the chip.

Finally, a solution containing RbN03 and Ba( 1*103)2 was applied to the patterned surface and, by cation replacement at the uncovered portions of the uppermost layer of the KTP chips, a series of integrated wave-guides, as described above, was formed. The remaining Ti portions were removed by etching. Each of the wave-guides, in the series thus formed, was approximately 10 mm long, of which the frequency doubling portion occupied approximately 6 mm and the DBR portion occupied approximately 4 mm. Some of the 9 wave-guides thus produced have complied with equation (4) above.

As another example, a series of integrated wave-guide units was designed in accordance with a fundamental infrared wavelength of 830 nm. As in the previous example, this series also included 9 integrated wave-guide units formed on a KTP chip. All of the Bragg reflector portions of this series were designed to have the same period length, of 2.7 μπι, adapted to reflect a wavelength of 830 nm (the fundamental wavelength). As in the previous example, the superperiods Lk of the frequency doubling portions included 12 sub-periods (i.e. k=12).

The first wave-guide in the series had a superperiod length of 43.2 pm divided into 12 sub-periods 41 of 3.6 pm length each. The difference in super-period length Lk between successive doubler portions was, again, 0.1 μπι, obtained by a modification of 0.1 i in one of their respective sub-periods 41. The series was produced in accordance with the method described in the 16622s03 previous example.

It should be appreciated that the Integrated waveguide units of the present invention provide a reliable and relatively inexpensive means for producing visible laser radiation embodied in a particularly compact device. The integration- of a doubler portion and a Bragg reflector portion into one unit, in the production stage, considerably reduces the sensitivity to ambient conditions, such as temperature, and obviates the need for post-production matching of the two portions. Furthermore, the optical wave-guiding continuum formed by the integration of the two portions avoids loss of energy, which commonly occurs at the interface of two optical elements.

It should also be appreciated that such an integrated waveguide is virtually unaffected by mechanical vibrations in the laser system. A frequency-converted laser apparatus using an integrated unit, in accordance with the present invention, will generally be less sensitive to small variations in the distance between the laser diode and the waveguide, compared to prior art systems which do not use a Distributed Bragg reflector. This is because control of the locked fundamental frequency "fo" is achieved by adjusting the peak of reflection of the DBR portion, which is independent of the distance between the diode and the waveguide .

It should be appreciated that the present invention is not limited what has been thus far described with reference to preferred embodiments of the invention. Rather, the scope of the present invention is limited only by the following claims:

Claims (30)

16622s03 C L A I M S

1. A frequency-converted laser apparatus comprising: a primary laser oscillator operative to emit primary radiation; a Bragg reflector adapted to reflect part of the primary radiation, within a frequency range centered at a preselected fundamental frequency, back along an optical path into the primary oscillator, thereby to form an external laser cavity resonating at the preselected fundamental frequency; and a frequency converter located within the external laser cavity, on the optical path between the primary laser oscillator and the Bragg reflector, and operative to convert part of the primary radiation into frequency-converted secondary radiation.

2. A frequency-converted laser apparatus according to claim 1 wherein the primary laser oscillator comprises a reflective back facet and wherein the external laser cavity extends from the reflective back facet to the Bragg reflector.

3. A frequency-converted laser apparatus according to claim 1 or claim 2 wherein both the frequency converter and the Bragg reflector are formed on a crystalline material.

4. Apparatus according to claim 3 wherein the crystalline material is a KTP crystal.

5. Apparatus according to any of the preceding claims wherein the primary radiation emitted by the primary laser oscillator is in the infrared range.

6. Apparatus according to any of the preceding claims wherein the primary laser oscillator is a diode laser. 16622S03

7. Apparatus according to any of the preceding claims wherein the frequency-converted secondary radiation is in the visible range.

8. Apparatus according to any of the preceding claims wherein the frequency of the secondary radiation is higher than the fundamental frequency.

9. Apparatus according to claim 8 wherein the frequency of the secondary radiation is a second harmonic frequency equal to twice the fundamental frequency.

10. Apparatus according to any of the preceding claims wherein the frequency-converter comprises a plurality of periodic segments including sections of a first kind, having a first index of refraction, and sections of a second kind, having a second index of refraction, and wherein the first kind of sections have a crystal polarity opposite the crystal polarity of the second kind of sections.

11. Apparatus according to claim 10 wherein each periodic segment is a superperiod Lk comprising k sub-periods and wherein each sub-period comprises one section of the first kind followed by one section of the second kind.

12. Apparatus according to any of the preceding claims wherein the Bragg reflector comprises a plurality of periodic reflecting segments including reflecting sections of a first kind, having a first index of refraction, and reflecting sections of a second kind, having a second index of refraction.

13. Apparatus according to claim 12 wherein each periodic reflecting segment is a superperiod comprising j reflecting sub-periods and wherein each reflecting sub-period comprises one reflecting section of the first kind followed by one reflecting section of the second kind. 16622s03 105863/2

14. 1 . Apparatus according to any of the preceding claims wherein the frequency converter comprises an input face for receiving the primary radiation, and further comprising at least one lens, located between the primary oscillator and the frequency-converting portion, for focusing the primary radiation onto the input face of the frequency converter.

15. An integrated wave-guide unit comprising: a frequency converting portion adapted to receive primary radiation from a primary radiation source and, upon receipt of the primary radiation, to convert part of the primary radiation into frequency-converted secondary radiation; and a Bragg reflector portion adapted to receive radiation from the frequency-converting portion and to reflect part of the primary radiation, within a limited frequency range centered at a preselected fundamental frequency, back through the frequency converting portion, wherein said Bragg reflector portion is positioned relative to said frequency converting portion such that said frequency converting portion is intermediate to said primary radiation source and said Bragg reflector portion.

16. An integrated wave-guide unit according to claim 15 wherein the frequency converting portion and the Bragg reflector portion are both formed on a single chip of crystalline material.

17. An integrated wave-guide unit according to claim 16 wherein the single chip of crystalline material is a chip of KTP crystal.

18. An integrated unit according to any of claims 15-17 wherein the primary radiation is in the infrared range.

19. An integrated unit according to any of claims 15-18 wherein the primary radiation is the output of a diode laser.

20. A unit according to any of claims 15-19 wherein the frequency-converted secondary radiation is in the visible range.

21. A unit according to any of claims 15-20 wherein the frequency of the secondary radiation is higher than the fundamental frequenc .

22. A unit according to claim 22 wherein the frequency of the secondary radiation is a second harmonic frequency equal to twice the fundamental frequency.

23. A unit according to any of claims 15-22 wherein the frequency converting portion comprises a plurality of periodic segments including sections of a first kind, having a first index of refraction, and sections of a second kind, having a second index of refraction, and wherein the first kind of sections have an electric polarity opposite the electric polarity of the second kind of sections .

24. 2 . A unit according to claim 23 wherein each periodic segment is a superperiod comprising k sub-periods and wherein each sub-period comprises one section of the first kind followed by one section of the second kind.

25. A unit according to any of claims 15-24 wherein the Bragg reflector portion comprises a plurality of periodic reflecting segments including reflecting sections of a first kind, having a first index of refraction, and reflecting sections of a second kind, having a second index of refraction.

26. A unit according to claim 25 wherein each periodic reflecting segment is a superperiod _j comprising j reflecting sub-periods and wherein each reflecting sub-period comprises one reflecting section of the first kind followed by one reflecting section of the second kind. ^16622s03 105863/2

27. A frequency-converted laser apparatus comprising: a primary laser oscillator, having a reflective back facet and a front facet, operative to emit primary radiation through the front facet; and an integrated wave-guide unit according to any of claims 15- 26 having an input face, optically associated with the front facet of the primary oscillator, for receiving the primary radiation; whereby an external laser cavity, resonating at the fundamental frequency, is formed between the reflective back facet of primary oscillator and the Bragg reflector portion of the integrated wave-guide.

28. An apparatus according to claim 27 and further comprising at least one lens, located between the front facet of the primary oscillator and the input face of the integrated wave-guide unit, for focusing the primary radiation onto the input face of the integrated wave-guide.

29. Apparatus substantially as shown and described hereinabove.

30. Apparatus substantially as illustrated in any of the drawings . For the Applicant, Sanford T. Colb & C: 16622