NOVELTY - The optical bi-spectral detector comprises two elementary photoconductor detectors (1,1'), each capable to detect two wavelength ranges (lambda1 and lambda2), and the diffraction gratings (3,3') for coupling the light of first and second wavelength ranges, respectively, so that each elementary detector can detect the light of respective wavelength range. The elementary detectors are implemented in the same layer of photoconducting material, and in a set of layers forming quantum wells. A third elementary detector (2) separated by a common contact or an insulator layer (4) allows an elimination of background noise of two elementary detectors.    USE - In bi-spectral or bi-functional optical detectors which allow the detection of two wavelength ranges with elimination of background noise.    ADVANTAGE - Possible elimination of background noise of elementary detectors with diffraction gratings; the device can be utilized in three subtractive modes corresponding to three spectral dimensions, the two wavelength ranges and the sum, that is lambda1 lambda2, and lambda1+lambda2.    DETAILED DESCRIPTION - The two elementary detectors are implemented in an alternation of at least two types of quantum wells, each type allowing the detection of one wavelength, lambda1 or lambda2. A fourth elementary detector is separated by a contact layer or an insulator layer, and forms the same elementary detector with the third. The four elementary detectors allow the detection of the same wavelength ranges. The response of two elementary detectors is higher than that of the third and fourth so that the light energy is preferentially absorbed in the wavelength range for which it is photoconducting. The diffraction gratings are made of conducting material or are covered with a conducting material, and comprise two contacts (C1, C'1), a third contact (Cr) to the common layer (4), and a fourth contact (C2) to an ohmic contact layer (5). The contacts (C1, C'1, C2) are for the application of control voltages (+V1, +V2, -Vref), and the contact (Cc) for measuring the photoconduction of elementary detectors. In an array of elementary detectors the contact (C2) is common to all elementary detectors so that the reference voltage (-Vref) is applied to all of them, all first contacts (C1) are connected together so that the control voltage (V1) is applied to all first elementary detectors, and all second contacts (C'1) are connected together so that the control voltage (+V2) is applied to all second elementary detectors. The method for implementing the optical detector comprises the steps: the implementation of an ohmic contact layer (5) on the face of a transparent substrate, the implementation of a set of layers (2) constituting alternating quantum wells, the implementation of layers of second ohmic contact (4), the implementation of a set of layers constituting quantum wells, the implementation of diffraction gratings (3, 3') of physically different structure, the etching of layers to attain the first ohmic contact layer (5), the encapsulation of layers in an insulator layer (6), the etching of insulator layer to attain the diffraction gratings and the metallization of openings to obtain the contacts (C1, C'1), the making of an opening per pair of elementary detectors to attain the second ohmic contact layer (4) and carrying out metallization to obtain the contact (Cr), and the making of an opening through the insulator layer to attain the first ohmic contact layer (5) and carrying out metallization to obtain the contact (C2).    DESCRIPTION OF DRAWING(S) - The drawing is a cross-sectional view of the detector.    Elementary photoconductor detectors (1,1',2)    Diffraction gratings (3,3')    Ohmic contact layers (4,5)    Encapsulating insulator layer (6)    Contacts (C1, C'1, Cc, C2)