AN OPTICAL FILTER DEVICE

Abstract: An optical filter device comprises an active region (100) having an amplifying function, another active region (300) having a noise absorbing function, and a distributed feedback region (200) having a wavelength selecting function. In the optical filter device, a phase shift degree is varied by injecting electric current into the distributed feedback region so that a predetermined wavelength of an optical signal can be selected from wavelength multiplexed signals of as much as sixty or seventy channels.

Description: The invention relates to a tunable optical filter device, and more particularly to a tunable optical filter device in which a predetermined wavelength of an optical signal can be selected from optical wavelength multiplexed signals of as much as sixty or seventy channels. A conventional optical filter device is described on pages 123 to 126 of "OPTICS COMMUNICATION, Volume 10, number 2, February 1974" as entitled "Analysis of the transmission, reflection and noise properties of distributed feedback laser amplifiers". The optical filter device comprises a distributed feedback structure having a diffraction grating which is advantageous in regard to a wavelength selection and an optically integrating property as compared to a Fabry-Perot resonator having cleavage plains. Such an optical filter device is one of important devices which is widely applied to an optical communication apparatus, an optical switching means, an optical information processing apparatus and so on to select a predetermined wavelength of an optical signal from optical wavelength multiplexed signals. In each application, the optical filter device is required to have a satisfactory property of a wavelength selection and a variable and large tuning range. The optical filter device is further desired to be a transmitting type of a wavelength selection filter device, through which a predetermined wavelength of an optical signal is passed because it is inevitable to be included in an optical integrated circuit. In regard to the aforementioned transmitting type of a wavelength selection filter device, several devices have been studied. Among them, an optical filter device which comprises an optical feedback structure having a wavelength selection property in an optical amplifying device utilizing a semiconductor active layer is expected to be put into a practical use because the tuning of a wavelength selection is varied dependent on a density of carriers injected into the active layer and it is suited to be included in an optical integrated circuit. Especially, an optical filter device which comprises a distributed feedback structure having a diffraction grating as described in the aforementioned "OPTICS COMMUNICATIONS" is advantageous in regard to a wavelength selection and an optically integrating property as explained before. According to the conventional optical filter device, however, there is a disadvantage that an optical gain for a selected wavelength and an intensity of spontaneous emission light are changed in a case where a density of carriers injected into the active layer is adjusted for the tuning of a selected wavelength. As a result, a density of carriers injected into the active layer is limited to be adjusted in principle in obtaining predetermined properties of a wavelength selection and a ratio of signal to noise. For this reason, a variable width of a wavelength selection is as narrow as several Å so that the optical filter device is only used for selecting a predetermined wavelength of an optical signal from wavelength multiplexed signals of several channels. EP-A-187 662 relates to a monolithically integrated semiconductor optical device which aims at establishing good optical connection and sufficient electrical insulation between the integrated functional devices. The semiconductor optical device comprises a laser diode functioning as distributed feedback region, and a light modulator. Between the distributed feedback region and the light modulator a gap is provided. The distributed feedback region comprises an active layer. Consequently, the transmission characteristics of the semiconductor optical device, namely the transmission factor and the transmission bandwidth are changed depending on the intensity of the light input. It is an object of the invention to provide a tunable optical filter device in which a tuning range of a selected wavelength is as broad as several tens Å. It is another object of the invention to provide a tunable optical filter device which is used for selecting a predetermined wavelength of an optical signal from wavelength multiplexed signals of as much as several tens channels. These objects are achieved by the tunable optical filter device according to the claims. The invention will be explained in more detail in conjunction with following drawings wherein, Fig. 1 is a perspective view showing an optical filter device in an embodiment according to the invention, and Fig. 2 is an explanatory diagram showing light transmitting properties of the optical filter device in the embodiment according to the invention. In Fig. 1, there is shown an optical filter device in an embodiment according to the invention. The optical filter device will be explained in accordance with a following fabricating procedure. At first, a λ/4 shifted diffraction grating having a period of 2400Å is formed at a DFB region 200 on an n-InP substrate 11 where λ is a wavelength of light propagated therein. Next, an optical guiding layer 12 of non-doped InGaAs 12 having λg of 1.3µm and a thickness of 0.3µm which is transparent for light of 1.5µm band for the guiding of the light, a buffer layer 13 of n-InP having a thickness of 0.1µm, a non-doped active layer 14 having λg of 1.63µm and a thickness of 0.1µm for amplifying light of 1.5µm band, and a cladding layer 15 of p-InP having a thickness of 0.2µm are grown sequentially on the substrate 11 in a first liquid phase epitaxy. The cladding layer 15 and the active layer 14 are then etched to be removed selectively at the DFB region 200 and a noise absorption region 300. At the present stage, the etching is stopped in the presence of the buffer layer 13 so that the buffer layer 13 and the optical guiding layer 12 remain unchanged. Thereafter, a non-doped active layer 21 having λg of 1.4µm and a thickness of 0.1µm, and a cladding layer of p-InP 17 having a thickness of 0.2 µm are grown sequentially at the noise absorption region 300 on the buffer layer 13 in a second liquid phase epitaxy. The active layer 21 is transparent for light of 1.5µm, and has a function to absorb light of 1.3µm band induced in the optical guiding layer 12 effectively. That is, a band gap energy of the active layer 21 is smaller than that of the optical guiding layer 12 and larger than that of the active layer 14. In a third liquid phase epitaxy, a cladding layer 16 is grown overall thereon. Then, a buried growth is performed to provide a buried structure in a fourth liquid phase epitaxy after a mesa etching is finished. As clearly understood from the illustration therein, a double channel planar buried structure is utilized in the embodiment. In the double channel planar buried structure, 31 and 33 are p-InP layers, 32 is a p-InP cladding layer (equal to the cladding layers 15 and 35), 34 is an n-InP layer, 36 is a p-InP layer, 37 is a InGaAsP cap layer, and 38 is a SiO₂ film. Next, electrodes 22 and 23 are provided on the back surface of the substrate 11 and the top surface of the grown layers respectively, and grooves 24 and 25 each having a width of 20µm are provided except for a central mesa portion between an amplifying region 100 and the DFB region 200 and between the DFB region 200 and the noise absorption region 300 so that electrical isolations are obtained therebetween. Finally, SiN films 18 are provided on both outer plains of the device by use of a plasma CVD means so that a reflectivity is decreased down to less than one percent (1%) thereon. In the embodiment, lengths of the amplifying regions 100, the DFB region 200, and the noise absorption region 300 are 100 µm, 500 µm, and 100 µm, respectively. Fig. 2 shows light output spectra of the optical filter device which is fabricated as described in Fig. 1. In a case where electric current of 50mA is injected into the amplifying region 100, a light extinction ratio of more than 20 dB is obtained, and a bandwidth is 0.5Å at an attenuation of 10dB. Further, a transmitting wavelength is 1.5563µm in a case where no electric current is injected into the DFB region 200 and the noise absorption region 300, while a transmitting wavelength is shifted to be 1.5505µm by 58Å in a case where electric current of 80mA is injected into the DBR region 200. As understood from the illustration in Fig. 2, a predetermined wavelength of an optical signal can be selected from optical wavelength multiplexed signals by a wavelength interval of 0.5Å in a half band of a wavelength stopband, that is, in a band of approximately 30Å. In other words, the optical filter device can be used for selecting a predetermined wavelength of an optical signal from optical wavelength multiplexed signals of as much as sixty channels. Next, operation of an optical filter device according to the invention will be explained in more detail. In an optical transmitting property of an optical waveguide having a distributed feedback structure, a stopband of approximately several tens Å is formed with the center at Bragg wavelength determined in accordance with an optical period of a diffraction grating of the distributed feedback structure when optical phases of a distributed feedback region are aligned to each other in a case where there is no optical gain in a wavelength passband. On the other hand, in a case where optical phases are deviated in the distributed feedback region on both sides of the center thereof by π, that is, in the so-called λ/4 shifted structure in which a pitch of a diffraction grating is deviated by λ/4 where λ is a wavelength of light to be propagated in the device, a stopband is splitted to produce a narrow wavelength passband of less than approximately 1 to 2Å in the center of the aforementioned stopband of approximately several tens Å. In a case where an optical waveguide which comprises a distributed feedback region having a λ/4 shifted structure is fabricated by a semiconductor of a composition for a wavelength shorter than a transmitting wavelength, this results in no optical gain therein. For this reason, a wavelength selection width of less than approximately 1 to 2Å is obtained with the center at Bragg wavelength in accordance with the injection of carriers thereinto, and there is expected no deterioration in a ratio of signal to noise caused by spontaneous emission light. Further, a phase shift amount can be adjusted by injecting electric current into a phase shift region from the external. As a result, a transmitting type of an optical filter device in which a variable tuning can be performed by selecting a predetermined wavelength of an optical signal in a wavelength range of several to several tens Å, the center of which is Bragg wavelength, for the reason why transmitting properties are varied in accordance with a phase shift amount as shown in Fig. 2. That is, a variable wavelength width is larger by changing a phase shift amount in the invention than that in a fixed phase shift amount. In the invention, an active region and distributed feedback region are optically combined to provide an optical filter device having an amplifying function because there is no optical gain in the aforementioned optical waveguide. That is, an optical filter device having an amplifying function is obtained in accordance with a structure in which light is transmitted through a distributed feedback region after the light is injected into an active region. In the invention, further, attention has been paid to following two points. One is that non-reflection or anti-reflection structures must be provided on both plains or facets of the optical filter device. One reason is that oscillation is produced therein unless the anti-reflection structures are provided. The other is that spontaneous emission is produced to provide noise source therein if electric current is injected into the distributed feedback region. In this respect, a second active region having a band gap energy smaller than that of the distributed feedback region and larger than that of the active region having an amplifying function is provided in an output region of the optical filter device. If no electric current is injected into the second active region, spontaneous emission light induced in the distributed feedback region is absorbed in the second active region so that noise is reduced in the optical filter device. In the embodiment as shown in Fig. 1, the materials and compositions may be replaced by other materials and compositions. Further, a phase shift degree may be controlled from the external so that a tuning wavelength region can be expanded by more than five Å. As a result, an optical filter device according to the invention can be used for selecting a predetermined wavelength of an optical signal from wavelength multiplexed signals of as much as approximately seventy channels. Still further, a diffraction grating having a phase shift may be replaced by a phase shift structure in which a uniform diffraction grating is formed and a width and thickness of an optical wavelength are changed, an optical waveguide structure is not limited to a planar structure or a burried structure, but may be a structure having an optical waveguide function, and the anti-reflection structure may be a window structure or a multi-layer coated structure.

Claims: A tunable optical filter device having a light input and a light output end and comprising a substrate supporting: two active regions comprising a light guiding zone, one of said active regions (100) being positioned at the light input side and another of said regions (300) being positioned at the light output side, a distributed feed back region (200) having a light guiding zone and being positioned between said active regions, so that all light guiding zones are aligned in a row; electrodes provided on the back surface of the substrate (11) opposed to the active regions and on the surfaces of each of the two active regions (100, 300) and the distributed feed back region opposing the substrate; and anti-reflection structures (18) provided at the end facets of the active regions (100, 300) at the input end and the output end of the filter device, respectively. An optical filter device according to claim 1, wherein a bandgap energy of an active layer (21) in said active region (300) which is positioned on the light output side is larger than a bandgap energy of an active layer (14) in the other active region (100) which is positioned on the light input side. An optical filter device according to claim 1 or 2, wherein said two active regions (100, 300) and said distributed feedback region (200) comprise said substrate (11) as a common substrate, a common optical guiding layer (12), and a common buffer layer (13), and said distributed feedback region (200) is provided with a diffraction grating having a phase shift in the center thereof which is formed in an intermediate layer between said substrate (11) and said optical guiding layer (12). An optical filter device according to claims 1 to 3, wherein said distributed feedback region (200) is electrically isolated from said two active regions (100, 300) by grooves (24, 25).

Inventors: NUMAI, TAKAHIRO
Applicants: NEC CORPORATION
Application year: 1988
Main classification: G02B6
IPC codes: H01S5, G02B6, G02F1, H01S3, H01L31