Distributed reflector and wavelength-tunable semiconductor laser

Abstract: A semiconductor laser comprises first and second distributed Bragg reflector diffraction gratings (10a,10b) being intervened by an active region having no diffraction grating; said first and second diffraction gratings (10a,10b) each have a plurality of repeating unit regions repeated at respective periods, thus forming respective modulation periods (Mf,Mr); in each of the repeating unit regions the length of the single period varies in the direction in which light transmits, continuously or at intervals, thereby varying the optical reflectivity of the diffraction gratings (10a,10b) depending on its position in each of its said repeating unit regions; said first and second diffraction gratings reflect light having wavelengths (λ 1 ,λ 2 ,.. and λ' 1 ,λ 2 ,...) slightly differing from each other; and said diffraction grating extends by at least two modulation periods. The application of voltages on said diffraction gratings enables coarse and fine tuning of lasing the wavelength.

Description: The present invention relates to a distributed reflector and a semiconductor laser with such a distributed reflector useful in optical transmission field, and more particularly to a wavelength-tunable semiconductor laser with a distributed reflector which is suitable as a light source for transmission, a tunable light source for synchronous rectification, and a light source for photometry in optical wavelength (frequency) multiplexing communication systems. As a wavelength-tunable light source, distributed reflector type semiconductor lasers have been studied widely. Fig. 1A is a schematic cross-sectional view showing a basic structure of a first conventional example of a wavelength-tunable distributed reflector type semiconductor laser. (e.g., Y. Tohmori et al., Electronics Letters, Vol. 19, p.656 (1983)). In Fig. 1A, 2 is an active waveguide layer, 3 an inactive waveguide layer, 20 a diffractive grating, 101 an active region, 102 a front side distributed reflection region, 103 a rear side distributed reflection region, and 104 a phase adjustment region. In such a type of laser, the distributed reflectors on the both front and rear sides have identical peak wavelengths and can lase stably at a low threshold value when their respective reflectivities are high. However, in the first conventional example, the distributed reflector regions 102 and 103 each may be deemed to have a uniform pitch Λ, and a uniform equivalent refractive index, n eq , as illustrated in Fig. 1B. Hence, Bragg wavelength, λ B , at which a high reflectivity is obtained will be observed as a single line as illustrated in Fig. 1C. Tunable range of a lasing wavelength is an amount of variation Δλ (=2Λ·Δn eq ) from the Bragg wavelength, λ B , and depends on the amount of variation Δn eq of the equivalent refractive index, n eq , caused by current injected into the distributed reflector region. Incidentally, a maximum relative equivalent refractive index variation, Δn eq /n eq , is 1 % or less, and various workers on experiments on wavelength tunability with the above-mentioned conventional distributed reflector type semiconductor lasers (cf., e.g., Y. Kotaki, et al., Electronics Letters, Vol. 23, p.327 (1987)) remained to report wavelength tunable ranges on the order of 10 nm, which is insufficient for providing a light source for optical wavelength multiplexing communication systems. Recently, a distributed reflector as shown in Fig. 2A has been proposed which enables wavelength tuning within the range on the order of above 10 nm, which is referred to herein as reflector according to a second conventional example. In Fig. 2A, broken line indicates portions whose depiction has been omitted. In this case, the waveguide has a diffractive grating of a pitch of Λ, formed partially and periodically, thereby giving a plurality of reflection peaks in the vicinity of Bragg wavelength, λ B , which depends on the pitch, Λ, as illustrated in Fig. 2B. In a laser structure, one of the reflection peaks is selected electrically (i.e., rough adjustment of wavelength), and wavelength tuning is performed in the vicinity of the selected reflection peak (i.e., fine adjustment of wavelength) (cf. U. S. Patent No. 4,896,325 to L. A., Coldren et al.; and V. Jayaraman et al., LEOS, "91, paper SDL15.5, 1991). There is a report on wavelength tuning which says only on an experimental basis, rough adjustment on the order of 50 nm was made (V. Jayaraman et al., IEEE, 13th International Conference on Semiconductor Lasers, Post Deadline Papers PD-11). In the second conventional reflector; as will be understood from Fig. 2B, reflectivities decrease considerably as the wavelengths depart from Bragg wavelength, λ B . There has been no report on continuous operation of a wavelength-tunable semiconductor laser having the above-mentioned reflector according to the second conventional example while only pulse operation was made to confirm its action. With view to complying to expected rapid and great increase in the amount of communication and information in future, various investigations have been made on optical wavelength (frequency) multiplexing communication systems and there is a keen desire on the development of a light source with a wavelength tunability over a wide range as a light source for transmission, and synchronous rectification. Also, it is desired to realize a wavelength-tunable light source which can cover a wide band useful in the field of photometry. Under the circumstances, the present invention has been made, and it is an object of the present invention to provide a distributed reflector which has a plurality of sharp high reflectivity peaks distributed at the same spacing over a wide band. Another object of the present invention is to provide a wavelength-tunable semiconductor laser which solves the problems of the conventional wavelength-tunable semiconductor lasers and which enables a wide range wavelength tuning over about 100 nm or more which is by one digit larger than has been achieved conventionally in a single mode wavelength tuning, and which causes no increase in threshold value within the tunable wavelength range even when the distributed reflector region has a relative equivalent refractive index variation, Δn eq /n eq , that is on the same level as the conventional one (about 1 %). As a result of extensive investigation, it has now been found that the above object can be achieved by providing a device according to the appended claim 1. Further advantageous modifications are as set out in the dependend claims. According to the present invention, a semiconductor laser having a wavelength tunability which enables a wide range wavelength tuning over a gain band width of an active waveguide layer with a high controllability. The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings. Fig. 1A is a schematic cross-sectional view showing a basic structure of a wavelength-tunable distributed reflector type semiconductor laser according to a first conventional example; Fig. 1B is a schematic cross-sectional view showing a part of the wavelength-tunable distributed reflector type semiconductor laser shown in Fig. 1A; Fig. 1C is a graph illustrating relationship of wavelength vs. reflectivity of the wavelength-tunable distributed reflector type semiconductor laser shown in Fig. 1A; Fig. 2A is a schematic cross-sectional view showing a diffractive grating of a distributed reflector according to a second conventional example; Fig. 2B is a graph illustrating reflectivity characteristics of a distributed reflector according to a second conventional example; Fig. 3A is a schematic cross-sectional view showing a diffractive grating of a distributed reflector; Fig. 3B is an enlarged view showing details of a diffractive grating of a distributed reflector; Fig. 3C is a graph illustrating reflectivity characteristics of a distributed reflector; Fig. 4A is a schematic cross-sectional view showing a diffractive grating of a distributed reflector; Fig. 4B is an enlarged view showing details of a diffractive grating of a distributed reflector; Fig. 4C is a graph illustrating reflectivity characteristics of a distributed reflector; Fig. 4D is a schematic cross-sectional view showing diffractive grating of a distributed reflector; Fig. 5A is a schematic cross-sectional view showing a diffractive grating of a distributed reflector; Fig. 5B is an enlarged view showing details of a diffractive grating of a distributed reflector; Fig. 5C is a graph illustrating reflectivity characteristics of a distributed reflector; Fig. 6A is a schematic cross-sectional view showing a diffractive grating of a distributed reflector; Fig. 6B is an enlarged view showing details of a diffractive grating of a distributed reflector; Fig. 6C is a diagram illustrating a pattern of a diffractive grating of a distributed reflector; Fig. 6D is a graph illustrating positions of reflectivity peaks of a diffractive grating of a distributed reflector; Figs. 7A and 7B each are schematic diagrams illustrating positions of reflectivity peaks of a diffractive grating of a distributed reflector according to the present invention; Figs. 7C and 7D each are schematic diagrams illustrating positions of reflectivity peaks of diffractive gratings of two distributed reflectors according to the present invention; Fig. 8 is a schematic cross-sectional perspective view showing a wavelength-tunable semiconductor laser of the present invention; Fig. 9 is a graph illustrating lasing characteristics of a distributed reflector type semiconductor laser according to a first example of the present invention; Fig. 10A is a plan view showing a wavelength-tunable semiconductor laser according to a second example of the present invention; Fig. 10B is a cross-sectional view taken along the line A-A' in Fig. 10A, showing a wavelength-tunable semiconductor laser according to a second example of the present invention; Fig. 10C is a cross-sectional view taken along the line B-B' in Fig. 10A, showing a wavelength-tunable semiconductor laser according to a second example of the present invention; Fig. 11 is a graph illustrating lasing characteristics of a distributed reflector type semiconductor laser according to a first example of the present invention; Fig. 12 is a graph illustrating lasing characteristics of a distributed reflector type semiconductor laser according to a second example of the present invention; Figs. 13A, 13B and 13C each are graphs illustrating lasing characteristics of a distributed reflector type semiconductor laser according to a third example of the present invention; Fig. 14 is a schematic cross-sectional perspective view showing a distributed feed back type semiconductor laser according to a fourth example of the present invention; Fig. 15A is a schematic plan view showing a distributed feed back type semiconductor laser according to a fifth example of the present invention; Fig. 15B is a schematic cross-sectional view taken along the line A-A' in Fig. 15A, showing a distributed feed back type semiconductor laser according to a fifth example of the present invention; Fig. 15C is a schematic cross-sectional view taken along the line B-B' in Fig. 15A, showing a distributed feed back type semiconductor laser according to a fifth example of the present invention; Fig. 16 is a graph illustrating reflectivity characteristics of distributed reflectors of the present invention and a second conventional example for comparison. Hereinafter, explanations being useful for understanding the present invention will be made in detail with reference to the accompanying drawings. A distributed reflector includes a diffractive grating formed on an optical waveguide. The diffractive grating has a unit region repeated at a period Mf. This period Mf is sometimes referred to herein also as modulation period. In each period Mf, the pitch of the diffractive grating varies continuously or at intervals from Λ a to Λ b (provided that Mf > Λ a , Λ b ) as shown in Figs. 3A and 3B where broken lines indicate portions whose depiction has been omitted for convenience's sake. In this case, as shown in Fig. 3C where broken lines indicate portions whose depiction has been omitted for convenience's sake, the distributed reflector has reflectivity characteristics such that multiple sharp high reflectivity peaks appear in a band, W B , of from λ a (= 2 Λ a n eq ) to λb (= 2Λ b n eq ) at a period approximately defined by a wavelength spacing of Δλ f = λ 0 2 /2n eq M f (λ 0 = n eq (Λ a + Λ b )). Another distributed reflector, designed to obtain similar effects as described above with a uniform pitch, Λ, includes a diffractive grating formed on an optical waveguide and having a regular, periodical serrate pattern having a pitch of Λ as shown in Fig. 5A. Herein, as shown in Fig. 5A, the optical waveguide includes a plurality of repeating unit regions repeated at a period of Mf, in each of which the composition of the optical waveguide varies, in the direction in which light transmits, continuously or at intervals from ga to gb. This structure gives rise to a plurality of repeating unit regions repeated at a period of Mf, in each of which equivalent refractive index varies, in the direction in which light transmits, continuously or at intervals from na to nb correspondingly as shown in Fig. 5B where broken lines indicate portions whose depiction has been omitted for convenience's sake. The distributed reflector has reflectivity characteristics such that multiple sharp high reflectivity peaks appear in a wavelength band, W B , ranging from wavelengths λ a (= 2 Λ a n eq ) to λb (= 2Λ b n eq ) at a period approximately defined by a wavelength spacing of Δλ f = λ 0 2 /(n a + n b )M f (λ 0 = (n a + n b )Λ) as shown in Fig. 5C where broken lines indicate portions whose depiction has been omitted for convenience's sake. Still another distributed reflector includes a diffractive grating formed on an optical waveguide, in which the diffractive grating mostly comprises a regular, periodical pattern having a uniform pitch in which a plurality of phase shifts are introduced. More specifically, as shown in Fig. 6A, the diffractive grating has a repeating structure that includes a plurality of repeating unit regions repeated at a period of Mf. In each repeating unit region, the diffractive grating has a constant pitch, Λ, as shown in Fig. 6B. At a certain number of selected positions in a single repeating unit region, pitches are different from other positions, or the gratings at the selected positions have longer or shorter pitches than those at positions other than the selected ones. Before and after the selected positions, the phase of the refractive index of the diffractive grating changes to "-" or "+", thus forming a phase control region, as shown in Fig. 6B, in which portions indicated by symbols "p" and "m" indicate regions containing "+" and "-" phase shifts, respectively, and arrows indicate respective positions where phase shifts are present. Fig. 6C is an enlarged view showing portions of the diffractive grating indicated by symbols A, B and C, respectively, in Fig. 6B. In the portion A, the diffractive grating has a constant pitch. In the portion B, a grating having a smaller pitch than that of the portion A is formed for only a single pitch in a central position thereof. Peak positions of the diffractive grating after this point (toward the right hand side) are shifted forward (toward the left hand side) as compared with those of the diffractive grating in the portion A. This phase shift results in phase modulation of the diffractive grating, so that effects can be obtained which are equivalent to those obtainable with a shorter pitch of the diffractive grating. On the other hand, in the portion C, a grating having a larger pitch than that of the portion A is formed for only a single pitch in a central position thereof. Peak positions of the diffractive grating after this point (toward the right hand side) are shifted rearward (toward the right hand side) as compared with those of the diffractive grating in the portion A in contrast to the case of the portion B. As a result of this phase shift, effects can be obtained which are equivalent to those obtainable with a larger pitch of the diffractive grating. The spacing at which phase shifts are inserted varies depending on their position in each repeating unit region as illustrated in Fig. 6B. In addition, the phase shifts inserted in a region ranging from the central position toward the left hand side form "+" shifts while those in a region ranging from the central position toward the right hand side form "-" shifts, thus giving effects equivalent to those obtainable with a pitch increasing gradually in each repeating unit region. Fig. 6D is a graph plotting the position of the peak of an n-th diffractive grating in a repeating unit region vs. number of peaks, n. In Fig. 6D, solid line corresponds to the reflector described above first (hereinafter, referred to as reflector 1) in which the pitch varies in each repeating unit region, and a broken line indicates a diffractive grating of the instant described subject matter. In the diffractive grating of the reflector 1, the pitch decreases continuously and therefore the slope of the curve becomes gentler so that generally the curve represents a parabola. On the other hand, when the pitch of a diffractive grating is constant, the position of the peak increases linearly with n. In this case, while peak positions are on a straight line in a sub region where no phase shift is inserted since the pitch is constant, the slope changes discontinuously at an inserted phase shift. Generally, the straight lines bear a close resemblance to the curve of the diffractive grating of the reflector 1. Hence, if the size of the phase shift and the spacing at which phase shifts are inserted are set up small enough, the solid line and the broken line substantially coincide with each other so that the diffractive grating used in the reflector of the instant reflector can exhibit reflectivity characteristics equivalent to those of the diffractive grating of reflector 1. Therefore, the reflector of the instant described subject matter has reflectivity characteristics such that in the vicinity of Bragg wavelength approximately defined by the pitch Λ, there appear multiple sharp high reflectivity peaks at a period approximately defined by a wavelength spacing of Δλ f = λ 0 2 /2n eq M f . In the case of the reflector of the instant described subject matter, since the grating has generally a constant pitch and it is sufficient to insert phase shifts partly or at only selected positions, time required for forming a pattern of diffractive grating using electron beams can be reduced considerably as compared with the diffractive grating of the reflector 1 in which the pitch varies. In addition, in the reflector of the instant described subject matter, there is no need for modulating the coupling coefficient, K, of the reflector or the composition of the optical waveguide, which facilitates its fabrication in contrast to reflectors described above. Now, explanation will be made on wavelength-tunable semiconductor lasers of the present invention. The wavelength-tunable semiconductor laser of the present invention may be of a distributed reflection type or a distributed feed back type. In the both cases, the semiconductor laser includes two distributed reflectors which have different structures one from another. Wavelengths at high reflectivity peaks of one of the two reflectors used in a semiconductor laser of the present invention are named λ 1 to λ n as illustrated in Fig. 7A for convenience's sake. The reflectivity characteristics of the other of the two reflectors are such that multiple sharp high reflectivity peaks λ' 1 to λ' n (Fig. 7B) appear in a band ranging from wavelengths λ' a = 2 Λ' a n eq to λ'b = 2Λ' b n eq at a wavelength spacing of Δλ r = λ' 0 2 /2n eq M r (λ' 0 = n eq (Λ' a + Λ' b )). Here, the two distributed reflector regions include diffractive gratings which have different pitch modulation periods M f and M r (Mf ≠ M r ). In the wavelength-tunable semiconductor laser of the present invention, the two different distributed reflector regions are controlled electrically independently of each other so that one of wavelengths λ' 1 to λ' n can coincide with one wavelength λ i (i = 1 to n) among wavelengths λ 1 to λ n , and laser emission can occur only at a wavelength in the vicinity of λ i . Figs. 7C and 7D illustrate laser emissions at wavelengths around λ 1 and λ 2 (i.e., i = 1 or 2), respectively. In the case where λ 1 to λ n , λ' 1 to λ' n , and λ" 1 to λ" n , are set up so that gain band of the semiconductor can be covered, control of lasing wavelength covering the gain band can be realized. The above-described diffractive grating may be formed in a distributed reflector type semiconductor laser having an active region and two inactive regions sandwiching the active region, with the diffractive grating being formed in the inactive regions, or in a distributed feed back type semiconductor laser whose resonator is constructed by an active region alone, with the diffractive grating being formed in the active region. Furthermore, a phase adjustment region where no diffractive grating is formed may be provided in the inactive region of the optical waveguide of the distributed reflector type semiconductor laser, and the refractive index of the phase adjustment region may be controlled independently of the above-described distributed reflector region. This enables fine adjustment of lasing wavelength in the vicinity of the aforementioned wavelength λ i , so that lasing can occur over the entire band ranging from wavelengths λ 1 to λ 2 . Still further, a phase adjustment region where no diffractive grating is formed may be provided in the active region of the optical waveguide of the distributed feed back type semiconductor laser, and the refractive index of the phase adjustment region may be controlled independently of the aforementioned distributed reflector region. In this case, it is possible to achieve fine adjustment of lasing wavelength in the vicinity of the aforementioned wavelength λ i , so that lasing can occur over the entire band ranging from wavelengths λ 1 to λ 2 . The distributed reflector type semiconductor laser of the present invention may include two sets of comb-shaped electrodes above the two distributed reflectors, i.e., front and rear distributed reflectors, and current may be injected or voltage may be applied to each of them independently. This can change average equivalent refractive indices of the front and rear distributed reflectors independently so that lasing can occur at a wavelength in the vicinity of any selected wavelength λ i . Changing the average refractive indices of the front and rear distributed reflectors by the same amount simultaneously enables fine adjustment of lasing wavelength around the aforementioned wavelength λ i . In the distributed reflector type semiconductor laser, the rest two electrodes of the aforementioned comb-shaped electrodes may be earthed, and current may be injected or voltage may be applied to the electrodes. This enables fine adjustment of lasing wavelength around λ i . Furthermore, in the distributed reflector type semiconductor laser, it enables continuous tuning of lasing wavelength around the wavelength λ i to inject current or apply voltage simultaneously to the aforementioned earthed comb-shaped electrodes and the inactive region of the optical waveguide where no diffractive grating is present. Also the distributed feed back type semiconductor laser of the present invention may include two sets of comb-shaped electrodes above the front and rear distributed reflectors, and current may similarly be injected to each of the comb-shaped electrodes independently, with varying its intensity. As a result the average equivalent refractive indices of the front and rear distributed reflectors can be changed independently so that lasing can occur at a wavelength in the vicinity of any selected wavelength λ i . Simultaneous change in the average refractive indices of the front and rear distributed reflectors by the same amount makes it possible to conduct fine adjustment of lasing wavelength around the aforementioned wavelength λ i . In the distributed feed back type semiconductor laser, the rest two electrodes of the aforementioned comb-shaped electrodes may be earthed and current may be injected to the electrodes to thereby enable fine adjustment of lasing wavelength around λ i . Furthermore, in the distributed feed back type semiconductor laser, simultaneous change in current injected to the aforementioned earthed comb-shaped electrodes and that injected to the active region of the optical waveguide where no diffractive grating is present enables continuous tuning of lasing wavelength around the wavelength λ i . To control the refractive index of the resonator, the semiconductor laser has externally attached thereto an electrical control device which is electrically connected via a conductor to each of the electrodes provided on the semiconductor laser and provides controlled current or voltage. The electrical control device is not limited particularly, and any conventional ones can be used so far as they can provide controlled current or voltage. As described above, lasing can occur at any desired wavelength in a band ranging from wavelengths λ 1 to λ n . In addition, setting the wavelengths λ 1 to λ n , λ' 1 to λ' n , and λ" 1 to λ" n , at values such that gain band of the semiconductor laser can be covered enables lasing at any desired wavelength within the range of the gain band. EXAMPLES Hereafter, the present invention will be described in more detail by examples. However, the invention should not be construed as being limited thereto. In the following examples, electrical control is realized by the use of an electrical control device as described above. However, for simplicity's sake, description and depiction of the electrical control device are omitted. Example 1 Fig. 8 is a schematic cross-sectional perspective view showing a wavelength-tunable semiconductor laser of a distributed reflector type according to a first example of the present invention. In Fig. 8, reference numeral 1 denotes an n-type InP substrate, 2 an InGaAsP active waveguide layer having a bandgap wavelength of 1.55 µm, 3 an InGAsP inactive waveguide layer having a bandgap wavelength of 1.3 µm, 4 a p-type InP cladding layer, 5 a p + -type InGaAsP cap layer, 6 a p-type InP current block layer, 7 an n-type current block layer, 8 an n-type electrode, 9 a p-type electrode, 10a a first or front diffractive grating including repeating unit regions repeated at a period of Mf, each of the repeating unit regions having a pitch intermittently varying from Λ a to Λ b , 10b a second or rear diffractive grating including repeating unit regions at a period of M r , each of the repeating unit region having a pitch intermittently varying from Λ' a to Λ' b , 11 a connection portion connecting an active waveguide layer and an inactive waveguide layer, 30 grooves, 101 an active region, 102 a first or front distributed reflector region, 103 a second or rear distributed reflector region, and 104 a phase adjustment region. Here, the diffractive grating 10 is one according to the one firstly described above. However, diffractive gratings according to the ones described secondly and thirdly may also be used with similar effects. The distributed reflector type semiconductor laser of the instant example can be fabricated as follows. First, the active waveguide layer 2 and the inactive waveguide layer 3 are formed on the n-type InP substrate 1 by an organic metal gas phase epitaxial growth method. Then, a resist is coated on a surface of the inactive waveguide layer 3, and a pattern of a diffractive grating having a modulated pitch is transferred on the resist by an electron beam exposure. The diffractive gratings 10a and 10b are formed by etching using the transferred pattern as a mask. Then, in order to control transverse modes, the waveguides are processed into stripes. The p-type InP current block layer 6, the n-type current block layer 7, the p-type InP cladding layer 4, and the p + -type InGaAs cap layer 5 are formed one on another in this order. Thereafter, the p-type electrode 9 and the n-type electrode 8 are formed thereon. Further, the p-type electrode 9 and the p + -type InGaAsP cap layer 5 are partially removed to form grooves 30, which electrically separate the semiconductor into an active region 101 that includes the active waveguide layer 2, a first or front distributed reflector region 102 including the diffractive grating 10a, a second or rear distributed reflector region 103 including the diffractive grating 10b, a phase adjustment region 104 including the inactive waveguide layer that has no diffractive grating. The grooves 30 are above portions of the semiconductor laser corresponding to connection portions that connect adjacent two of these regions. In the diffractive grating used in the wavelength-tunable distributed reflector type semiconductor laser according to the instant example, the repeating unit region of which the pitch of the diffractive grating 10a intermittently varies from 2459 Å to 2389 Å is repeated at a period of 75 µm while the repeating unit region of which the pitch of the diffractive grating 10b intermittently varies from 2454 Å to 2385 Å is repeated at a period of 67.5 µm. The distributed reflector type semiconductor laser having the above-described construction lases upon application of current to the active region 101 thereof, and the lasing wavelength can be controlled by applying current or voltage separately to the distributed reflector regions 102 and 103, and the phase adjustment region 104. Fig. 9 is a graph illustrating change of lasing wavelength when current to be applied to the distributed reflector region 103 varies in a state where a constant current is applied to the active region 101, with applying no current to the distributed reflector region 102 and the phase adjustment region 104. As illustrated in Fig. 9, in the distributed reflector type semiconductor laser of the instant example, application of current to the distributed reflector region 103 results in change of lasing wavelength from 1.575 µm to 1.530 µm at an interval of about 50 µm, thus giving a maximum wavelength tuning of 450 Å. Then, application of independently controlled currents to the distributed reflector region 102 and the phase adjustment region 104, respectively, can vary lasing wavelength over the entire bandwidth of 450 Å. In the instant example, while explanation has been made on the case where the active waveguide layer and the inactive waveguide layer constitute a single semiconductor layer, the present invention is applicable to a laminated semiconductor including a plurality of semiconductor layers having different compositions such as an MQWS (multi-quantum well structure) laser. Table 1 shows relationship between the pitch of the diffractive grating used in the semiconductor laser of the instant example with Bragg wavelength when no control current is applied. Front DBR Region Rear DBR Region No. Pitch (Å) Bragg Wavelength (µm) No. Pitch (Å) Bragg Wavelength (µm) 1 2459 1.5748 1 2454 1.5719 2 2451 1.5696 2 2446 1.5662 3 2443 1.5645 3 2437 1.5605 4 2435 1.5594 4 2428 1.5549 5 2427 1.5543 5 2419 1.5493 6 2419 1.5494 6 2411 1.5438 7 2412 1.5444 7 2402 1.5383 8 2404 1.5394 8 2394 1.5329 9 2396 1.5345 9 2385 1.5275 10 2389 1.5296 Example 2 Figs. 10A to 10C are schematic views showing a wavelength-tunable distributed reflector type semiconductor laser according to a second example of the present invention. Fig. 10A is a schematic plan view showing the distributed reflector type semiconductor laser of the instant example. Fig. 10B is a schematic cross-sectional view taken along the line A-A', showing the distributed reflector type semiconductor laser shown in Fig. 10A, and Fig. 10C is a schematic cross-sectional view taken along the line B-B', showing the distributed reflector type semiconductor laser shown in Fig. 10A. In Figs. 10A to 10C, reference numerals 1 to 8, 10a, 10b, 11, 30, and 101 to 104 are the same as in Fig. 8, and 9a denotes a p-type electrode provided in the active region 101, 9b a p-type electrode provided in the phase adjustment region 104, 9c and 9d a set of comb-shaped p-type electrodes provided in the front distributed reflector region 102, 9e and 9f a set of comb-shaped p-type electrodes provided in the rear distributed reflector region 103, and 90 teeth of the comb-shaped electrodes. Here, the diffractive grating 10 is one according to the one firstly described above. However, diffractive gratings according to the ones described secondly and thirdly may also be used with similar effects. The distributed feed back type semiconductor laser of the instant example can be fabricated in a manner similar to Example 4 except that instead of the electrodes 9, the comb-shaped electrodes 9c and 9d, and 9e and 9f are formed in the front and rear distributed reflector regions 102 and 103, respectively. In the diffractive grating used in the wavelength-tunable distributed feed back type semiconductor laser according to the instant example, the repeating unit region of which the pitch of the diffractive grating 10a intermittently varies from 2459 Å to 2389 Å is repeated at a period of 75 µm while the repeating unit region of which the pitch of the diffractive grating 10b intermittently varies from 2454 Å to 2385 Å is repeated at a period 67.5 µm. Each comb-shaped electrode has a plurality of teeth 90 arranged at the same period as the pitch modulation period. That is, the individual electrodes (teeth) 90 of the comb-shaped electrodes 9c and 9d for the front distributed reflector region 102 are arranged at a period of 75 µm and the width w 1 of a single tooth 90 is about half this period (about 37.5 µm). The individual electrodes (teeth) 90 of the comb-shaped electrodes 9e and 9f are arranged at a period of 67.5 µm and the width w 2 of a single tooth 90 is about half this period (about 33.75 µm). The distributed feed back type semiconductor laser having the above-described construction lases upon application of current to all the regions thereof, and the lasing wavelength varies upon application of currents separately and independently to the distributed reflector regions 102 and 103, and the phase adjustment region 104. Fig. 11 illustrates the variation of lasing wavelength when current applied to the comb-shaped electrode 9e in the distributed reflector region 103 varies under the conditions that a constant current is applied to the active region 101, and no current is applied to the comb-shaped electrodes 9c and 9d in the front distributed reflector region 102, the comb-shaped electrode 9f in the rear distributed reflector region 103 and the p-type electrode 9b in the phase adjustment region 104. As illustrated in Fig. 11, in the distributed reflector type semiconductor laser of the instant example, application of current to the distributed reflector region 103 results in intermittent change of lasing wavelength from 1.575 µm to 1.530 µm at an interval of about 50 µm. Further, it is possible to conduct fine control of the lasing wavelength by independently applying fixed currents to the comb-shaped electrodes 9c and 9e, respectively, selecting one of different lasing wavelengths differing one from another at an interval of about 50 Å, electrically short-circuiting the comb-shaped electrodes 9d and 9f, and applying current to the short-circuited electrodes simultaneously. Fig. 12 is a graph illustrating relationship between variation of lasing wavelength and current. Solid lines in Fig. 12 represent behavior of wavelength when the current applied to the aforementioned short-circuited comb-shaped electrodes 9d and 9f varies. As illustrated in Fig. 12, with the distributed reflector type semiconductor laser, simultaneous application of current to the short-circuited comb-shaped electrodes 9d and 9f results in change in lasing wavelength by about 50 Å with a leap in wavelength. Further, broken lines in Fig. 12 represent behavior wavelength when current is applied to the p-type electrode 9b provided in the phase adjustment region 104, and the current applied to the short-circuited comb-shaped electrodes 9d and 9f varies. Thus, it is possible to conduct further fine control of the lasing wavelength by controlling the current injected to the p-type electrode 9b in the phase adjustment region. Adjustment of the currents applied to the p-type electrodes 9b to 9f, respectively, by the aforementioned procedure, enables coarse adjustment and fine adjustment of the lasing wavelength and selection of any desired lasing wavelength from over entire bandwidth of 450 Å. Example 3 Here, explanation will be made on a wavelength-tunable distributed reflector type semiconductor laser according to a third example of the present invention. In the distributed reflector type semiconductor lasers according to Examples 1 and 2, respectively, of the present invention, the pitch modulation period of the diffractive grating is the same as that at which the individual electrodes or teeth of the comb-shaped electrodes are arranged. When the period of repetition of the individual electrodes of the comb-shaped electrodes is smaller than the pitch modulation period of the diffractive grating, similar effects can be obtained as Examples 1 and 2 above. On the other hand, when the former is larger than the latter, different effects can be obtained. As an example of this, explanation will be made hereinbelow as to how to control lasing wavelength of a semiconductor laser which has the same structure as the wavelength-tunable semiconductor laser described in Example 2 as shown in Figs. 10A to 10C except that the widths of the individual electrodes of the comb-shaped electrodes 9c to 9f are twice as large as in the case of Example 2. Figs. 13A to 13C are graphs each illustrating reflectivity characteristics of the distributed reflector region of the semiconductor laser according to the instant embodiment. When no current is applied to the comb-shaped electrodes, the reflectivity characteristics of the distributed reflector region are as illustrated in Fig. 13A. That is, reflectivity peaks appear periodically at a wavelength spacing Δλ f approximately defined by the pitch modulation period M f of the diffractive grating. Here, application of current to one of a set of the comb-shaped electrodes whose individual electrodes or teeth formed above the diffractive grating at a period of 2M f results in variation of the refractive index at a period of 2M f , so that the distributed reflector can have reflectivity characteristics such that reflectivity peaks appear periodically at a wavelength spacing of Δλ f /2 as illustrated in Fig. 13B. Further, when the amount of current to be applied to the electrode increases, reflectivity characteristics changes as if the reflectivity peaks when no current is applied are shifted by an amount of Δλ f /2. Coarse adjustment of lasing wavelength can be realized at a wavelength spacing of Δλ f /2 based on this principle. In other words, the semiconductor lasers of Examples 1 and 2 enable coarse adjustment of a lasing wavelength at an interval of 50 Å while the semiconductor laser of the instant examples allows coarse adjustment of a lasing wavelength at an interval of 25 Å. Example 4 Fig. 14 is a schematic cross-sectional perspective view showing a wavelength-tunable distributed feed back type semiconductor laser according to a fourth example of the present invention. In Fig. 14, reference numeral 1 denotes an n-type InP substrate, 2 an InGaAsP active waveguide layer having a bandgap wavelength of 1.55 µm, 3a an InGAsP optical confinement layer having a bandgap wavelength of 1.3 µm, 4 a p-type InP cladding layer, 5 a p + -type InGaAsP cap layer, 6 a p-type InP current block layer, 7 an n-type current block layer, 8 an n-type electrode, 9 a p-type electrode, 10a a first or front diffractive grating including repeating unit regions repeated at a period of M f , each of the repeating unit regions having a pitch intermittently varying from Λ a to Λ b , 10b a second or rear diffractive grating including repeating unit regions at a period of M r , each of the repeating unit region having a pitch intermittently varying from Λ' a to Λ' b , 101 an active layer, 102 a first or front distributed reflector region, 103 a second or rear distributed reflector region, and 104 a phase adjustment region. Here, the diffractive gratings 10a and 10b are ones according to the one firstly described above. However, diffractive gratings according to the secondly and thirdly described ones may also be used with similar effects. Fabrication of the distributed reflector type semiconductor laser of the instant example will be described briefly. First, the active waveguide layer 2 and the optical confinement layer 3a are formed on the n-type InP substrate 1 by an organic metal gas phase epitaxial growth method. Then, a resist is coated on a surface of the optical confinement layer 3a, and a pattern of a diffractive grating having a modulated pitch is transferred on the resist by an electron beam exposure. The diffractive gratings 10a and 10b are formed by etching using the transferred pattern as a mask. Then, in order to control transverse modes, the waveguides are processed into stripes. The p-type InP current block layer 6, the n-type current block layer 7, the p-type InP cladding layer 4, and the p + -type InGaAs cap layer 5 are formed one on another in this order. Thereafter, the p-type electrode 9 and the n-type electrode 8 are formed thereon in this order. Further, the p-type electrode 9 and the p + -type InGaAsP cap layer 5 are partially removed to form grooves 30, which electrically separate the semiconductor into a first or front distributed reflector region 102 including the diffractive grating 10a, a second or rear distributed reflector region 103 including the diffractive grating 10b, a phase adjustment region 104 including the inactive waveguide layer that has no diffractive grating. The grooves 30 are above portions of the semiconductor laser corresponding to connection portions that connect adjacent two of these regions. In the diffractive grating used in the wavelength-tunable distributed feed back type semiconductor laser according to the instant example, the repeating unit region of which the pitch of the diffractive grating 10a intermittently varies from 2459 Å to 2389 Å is repeated at a period of 75 µm while the repeating unit region of which the pitch of the diffractive grating 10b intermittently varies from 2454 Å to 2385 Å is repeated at a period 67.5 µm. The distributed reflector type semiconductor laser having the above-described construction lases upon application of current to each of the regions thereof, and the lasing wavelength can be controlled by varying the currents independently of each other. Fig. 9 is a graph illustrating variation of lasing wavelength when the current applied to the rear distributed reflector region 103 varies in a state where a constant current is applied to each of the front distributed reflector region 102 and the phase adjustment region 104. As illustrated in Fig. 9, in the distributed feed back type semiconductor laser of the instant example, application of current to the distributed reflector region 103 results in change of lasing wavelength from 1.575 µm to 1.530 µm at an interval of about 50 µm, thus giving a maximum wavelength tuning of 450 Å. Then, application of independently controlled currents to the distributed reflector region 102 and the phase adjustment region 104, respectively, can vary lasing wavelength over the entire bandwidth of 450 Å. In the instant example, while explanation has been made on the case where the active waveguide layer and the inactive waveguide layer constitute a single semiconductor layer, the present invention is applicable to laminated semiconductor including a plurality of semiconductor layers having different compositions such as MQWS (multi-quantum well structure) laser. Example 5 Figs. 15A to 15C are schematic views showing a wavelength-tunable distributed feed back type semiconductor laser according to a fifth example of the present invention. Fig. 15A is a schematic plan view showing the distributed feed back type semiconductor laser of the instant example. Fig. 15B is a schematic cross-sectional view taken along the line A-A', showing the distributed feed back type semiconductor laser shown in Fig. 15A, and Fig. 15C is a schematic cross-sectional view taken along the line B-B', showing the distributed reflector type semiconductor laser shown in Fig. 15A. In Figs. 15A to 15C, reference numerals 1 to 8, 10a, 10b, 11, 30, and 101 to 104 are the same as in Fig. 14, and 9b denotes a p-type electrode provided in the phase adjustment region 104, 9c and 9d a set of comb-shaped p-type electrodes provided in the front distributed reflector region 102, 9e and 9f a set of comb-shaped p-type electrodes provided in the rear distributed reflector region 103, and 90 tooth of comb-shaped electrode. Here, the diffractive grating 10 is one according to the one firstly described above. However, diffractive gratings according to the ones described secondly and thirdly may also be used with similar effects. The distributed reflector type semiconductor laser of the instant example can be fabricated in a manner similar to Example 1 except that instead of the electrodes 9, the comb-shaped electrodes 9c and 9d, and 9e and 9f are formed in the front and rear distributed reflector regions 102 and 103, respectively. In the diffractive grating used in the wavelength-tunable distributed reflector type semiconductor laser according to the instant example, the repeating unit region of which the pitch of the diffractive grating 10a intermittently varies from 2459 Å to 2389 Å is repeated at a period of 75 µm while the repeating unit region of which the pitch of the diffractive grating 10b intermittently varies from 2454 Å to 2385 Å is repeated at a period 67.5 µm. Each comb-shaped electrode has a plurality of teeth 90 arranged at the same period as the pitch modulation period. That is, the individual electrodes (teeth) 90 of the comb-shaped electrodes 9c and 9d for the front distributed reflector region 102 are arranged at a period of 75 µm and the width w 1 of a single tooth 90 is about half this period (about 37.5 µm). The individual electrodes (teeth) 90 of the comb-shaped electrodes 9e and 9f are arranged at a period of 67.5 µm and the width w 2 of a single tooth 90 is about half this period (about 33.75 µm). The distributed reflector type semiconductor laser having the above-described construction lases upon application of current to the active region 101 thereof, and the lasing wavelength varies upon application of current or voltage separately and independently to the distributed reflector regions 102 and 103, and the phase adjustment region 104. Fig. 11 illustrates the variation of lasing wavelength when current applied to the comb-shaped electrode 9e in the distributed reflector region 103 varies under in a state where a constant current is applied to the comb-shaped electrodes 9c and 9d in the front distributed reflector region 102 and the comb-shaped electrode 9f in the rear distributed reflector region 103, and the p-type electrode 9b in the phase adjustment region 104 to lase. As illustrated in Fig. 11, in the distributed feed back type semiconductor laser of the instant example, application of current to the distributed reflector region 103 results in intermittent change of lasing wavelength from 1.575 µm to 1.530 µm at an interval of about 50 µm. Further, it is possible to conduct fine control of the lasing wavelength by independently applying a current of a fixed intensity to the comb-shaped electrode 9e, selecting one of different lasing wavelengths differing one from another at an interval of about 50 Å, electrically short-circuiting the comb-shaped electrodes 9d and 9f, and applying current to the short-circuited electrodes and varying the intensity of the current simultaneously. Fig. 12 illustrates relationship between variation of lasing wavelength and current. Solid lines in Fig. 12 represent behavior of wavelength when the current applied to the aforementioned short-circuited comb-shaped electrodes 9d and 9f varies. As illustrated in Fig. 12, with the distributed feed back type semiconductor laser, simultaneous application of current to the short-circuited comb-shaped electrodes 9d and 9f results in variation in lasing wavelength by about 50 Å with a leap in wavelength. Further, broken lines in Fig. 12 represent behavior of wavelength when current is applied to the p-type electrode 9b provided in the phase adjustment region 104, and the current applied to the short-circuited comb-shaped electrodes 9d and 9f varies by varying the intensity of the current applied to the electrode 9b. Thus, it is possible to conduct further fine control of the lasing wavelength by controlling the current to the p-type electrode 9b in the phase adjustment region. Adjustment of the currents applied to the p-type electrodes 9b to 9f, respectively, by the aforementioned procedure, enables coarse adjustment and fine adjustment of the lasing wavelength and selection of any desired lasing wavelength from over entire bandwidth of 450 Å. Example 6 A wavelength-tunable distributed reflector type semiconductor laser according to a sixth example of the present invention will be described below. In the distributed reflector type semiconductor lasers according to Examples 4 and 5, respectively, of the present invention, the pitch modulation period of the diffractive grating is the same as that at which the individual electrodes or teeth of the comb-shaped electrodes are arranged. When the period of repetition of the individual electrodes of the comb-shaped electrodes is smaller than the pitch modulation period of the diffractive grating, similar effects can be obtained as Examples 4 and 5 above. On the other hand, when the former is larger than the latter, different effects can be obtained. As an example of this, explanation will be made hereinbelow as to how to control the lasing wavelength of a semiconductor laser which is of the same structure as the wavelength-tunable semiconductor laser described in Example 5 as shown in Figs. 15A to 15C except that the widths of the individual electrodes of the comb-shaped electrodes 9c to 9f are twice as large as those in the case of Example 5. Figs. 13A to 13C each illustrate reflectivity characteristics of the distributed reflector region of the semiconductor laser according to the instant example. When no current is applied to the comb-shaped electrodes, the reflectivity characteristics of the distributed reflector region are as illustrated in Fig. 13A. That is, reflectivity peaks appear periodically at a wavelength spacing Δλ f approximately defined by the pitch modulation period M f of the diffractive grating. Here, application of current to one of a set of the comb-shaped electrodes whose individual electrodes or teeth formed above the diffractive grating at a period of 2M f results in variation of the refractive index at a period of 2M f , so that the distributed reflector can have reflectivity characteristics such that reflectivity peaks appear periodically at a wavelength spacing of Δλ f /2 as illustrated in Fig. 13B. Further, when the amount of current to be applied to the electrode increases, reflectivity characteristics varies as if the reflectivity peaks when no current is applied are shifted by an amount of Δλ f /2. Coarse adjustment of lasing wavelength can be realized at a wavelength spacing of Δλ f /2 based on this principle. In other words, the semiconductor lasers of Examples 4 and 5 enable coarse adjustment of a lasing wavelength at an interval of 50 Å while the semiconductor laser of the instant examples allows coarse adjustment of a lasing wavelength at an interval of 25 Å. Example 7 Fig. 16 is a graph illustrating relationship between number of peaks and reflection factor of a distributed reflector of a distributed reflector, comparing the present invention with a second conventional example. Horizontal axis indicates number of reflectivity peaks and vertical axis reflectivity of a central peak. Curve 40 represents reflectivity characteristics of the distributed reflector of the present invention while Curve 41 relates to the reflectivity characteristics of the second conventional example. In the both cases, depth of diffractive grating is constant. No remarkable decrease in reflectivity despite increase in the number of peaks along the horizontal axis indicates that lasing is possible within a wavelength range wide enough to cover the number of reflectivity peaks. In the present invention, reflection factor is large enough when the number of reflectivity peaks is 10 or more. On the contrary, in the second conventional example, reflection factor decreases to a considerably low level when the number of reflectivity peaks approaches 10, thus making it impossible to realize lasing at low threshold, and at worst lasing stops. Therefore, such a distributed reflector as that of the second conventional example, laser emission in a broader wavelength band was difficult. The present invention has been described in detail in the foregoing, and it will now be apparent from this to those skilled in the art that changes and modifications may be made without departing from the present invention in its broader aspects, and it is the intention, therefore, in the appended claims to cover all such changes and modifications as fall within the scope of the invention. The distributed reflector includes a substrate(1) and at least one optical waveguide formed on the substrate and having a refractive index larger than the substrate and at least one optical confinement layer(3) having a refractive index smaller than the optical waveguide layer. A diffractive grating(10a or 10b) is formed in at least one layer(2 or 3) constituting the optical waveguide. The diffractive grating has a structure of which at least one parameter defining optical reflectivity varies depending on its position, and is formed continuously for at least two periods, the period being approximately defined by the length of repeating unit region. The parameter may be pitch, coupling coefficient, bandgap composition, phase shift, etc. The semiconductor laser includes the distributed reflector which may be of a distributed reflector type or distributed feed back type and has distributed reflector regions(102, 103) and a phase adjustment region(104). The laser also has a cladding layer(4), a cap layer(5), current block layers(6, 7), electrodes(8, 9), connection portions(11), and grooves(30). Application of current or voltage to the refractive index of the active region(101) or inactive region(102, 103, 104) in which a diffractive grating is present or absent adjusts the refractive index thereof and enables coarse and fine adjustment of lasing wavelength. Comb-shaped electrodes may be used.

Claims: A semiconductor laser, comprising: a substrate (1); at least one optical waveguide layer (2, 3) provided on said substrate and having an optical refractive index greater than the substrate (1); at least one optical confinement layer (3a) provided on said substrate (1) and having an optical refractive index smaller than the optical waveguide layer (2, 3); and a diffraction grating (10a, 10b) provided in at least one of the layers constituting said optical waveguide (2, 3) and said at least one optical confinement layer (3a) and having a period; said semiconductor laser (2, 3, 3a, 10a, 10b) reflecting light having a wavelength approximately defined by Bragg conditions; said semiconductor laser being characterized in that said semiconductor laser comprises first and second diffraction gratings (10a, 10b), said first and second diffraction gratings (10a, 10b) being intervened by a region where said at least one of the layers constituting said optical waveguide (2, 3) and said at least one optical confinement layer (3a) has no diffraction grating and where said at least one optical waveguide is an active waveguide (2) ; said first and second diffraction gratings (10a, 10b) each have a plurality of repeating unit regions repeated at respective periods, thus forming respective modulation periods (Mf, Mr); in each of the repeating unit regions the length of the single period varies in the direction in which light transmits, continuously or at intervals, thereby varying the optical reflectivity of the diffraction gratings (10a, 10b) depending on its position in each of said repeating unit regions; said first and second diffraction gratings reflect light having wavelengths slightly differing from each other; and said diffraction grating extends by at least two modulation periods. A semiconductor laser according to claim 1, characterized in that said substrate (1) is a semiconductor substrate. A distributed reflector semiconductor laser according to claim 1 or 2, characterized in that said at least one optical waveguide layer (2, 3) comprises an active waveguide layer (2) defining an active waveguide region and first and second inactive waveguide layers (3) optically connected to said active waveguide layer (2) at its both ends, defining inactive regions having respective refractive indices; said first and second diffraction gratings (10a, 10b) are provided in said first and second inactive regions; said active waveguide region has an optical gain to light having a wavelength which said distributed reflector reflects; and said active waveguide region performs an optical amplification action and said distributed reflector performs a reflection action, so that lasings at respective reflection wavelengths in said distributed reflector are caused; and said distributed reflector semiconductor laser comprises means (8, 9) for electrically controlling said respective refractive indices of said inactive regions. A distributed reflector semiconductor laser according to claim 3, characterized in that said means (8, 9) for electrically controlling said respective refractive indices is a means for injecting current in said first and second inactive regions in which said first and second diffraction gratings (10a, 10b) are present. A distributed reflector semiconductor laser according to claim 3, characterized in that means (8, 9) are additionally provided for electrically controlling the refractive index of said active waveguide region comprising means for injecting current in said active waveguide region. A distributed reflector semiconductor laser according to claim 4, characterized by further comprising two sets of independently arranged comb-shaped electrodes (9c, 9d, 9e, 9f) in said inactive regions in which said first and second diffraction gratings (10a, 10b) are present. A distributed feedback semiconductor layer, according to claim 1 or 2, characterized in that said at least one optical waveguide layer (2, 3) comprises an active waveguide layer (2) having a refractive index and a light confinement layer (3a) which confines light from said active waveguide layer (2) ; said first and second diffraction gratings (10a, 10b) are provided in a portion of said light confinement layer (3a); said active waveguide has an optical gain to light having a wavelength which said semiconductor laser reflects; and said active waveguide performs an optical amplification action and said semiconductor laser performs a reflection action, so that lasings at respective reflection wavelengths are caused; and said distributed feedback semiconductor laser comprises means (8, 9) for electrically controlling the refractive index of said active waveguide region. A distributed feedback semiconductor laser according to claim 7, characterized in that said means (8, 9) for electrically controlling said refractive index is a means for injecting current in said active waveguide region through said confinement layer in which said first and second diffraction gratings (10a, 10b) are present.

Inventors: ISHII HIROYUKI, KANO FUMIYOSHI, TAMAMURA TOSHIAKI, TOHMORI YUICHI, YOSHIKUNI YUZO, ISHII, HIROYUKI, KANO, FUMIYOSHI, TAMAMURA, TOSHIAKI, TOHMORI, YUICHI, YOSHIKUNI, YUZO
Applicants: NIPPON TELEGRAPH & TELEPHONE, NIPPON TELEGRAPH AND TELEPHONE CORPORATION
Application year: 1993
Main classification: H01S5
IPC codes: G02B6, H01S5