US 6,320,179 B1 · Device noise measurement system

Abstract

A system and method for determining noise power levels across a range of frequencies of the optical signal output from a device operating at a specified optical power and drive current and, in particular, for determining relative intensity noise values (RIN). The system may comprise a reference laser for generating an optical reference signal and a photodetector for converting the optical signal output from the reference laser and the device under test into equivalent electrical signals. The system also comprises means for measuring ac and dc components of the electrical signals output from the photodetector. The system may be used to measure RIN levels to an accuracy of less than 1 dB. Such measurements are of particular importance in the field of fiber optic telecommunications. The reference laser is shot noise limited in the frequency range of interest, typically between 20 MHz and 20 GHz, and has a narrow linewidth, typically less than 5 kHz. A shot noise limited reference laser signal may be achieved by means of optically attenuating a higher power reference laser signal to the required lower optical power. The incorporation of a reference laser enables a simple calibration process to take account of factors such as the responsivity, gain and resolution bandwidth of the system apparatus across the whole measurement range.

Claims21

Dependent claims

A system for measuring one or more noise contributions to an optical output signal, OPTDUT, (3) from a device (1) comprising;

means (19) for generating a broadband, substantially shot noise limited optical reference signal, OPTREF, (21),
detection means (10) for receiving the optical signals, OPTDUT and OPTREF, (3, 21) and converting said optical signals into equivalent electrical signals, ELECDUT and ELECREF respectively, each comprising an ac component and a dc component (13), wherein the dc components (13) of the electrical signals, ELECDUT and ELECREF, are substantially equal,
means (12) for separating the ac components and the dc components (13) of the electrical signals, ELECDUT and ELECREF,
means (14) for measuring the dc components of the electrical signals, ELECDUT and ELECREF,
means (16) for amplifying the ac components of the electrical signals and
means (18) for measuring the amplified ac components of the electrical signals at a plurality of frequencies, whereby the measurement of at least one of the ac components or dc components (13) of the electrical signals, ELECDUT and ELECREF, provides an indication of one or more of the noise contributions to the optical output signal, OPTDUT (3).

3 more elements

A method for determining the device noise power contribution, NL, to the total noise power, NT, of an optical signal (3), OPTDUT, output from a device (1), having a wavelength λ, using apparatus contributing a thermal noise power, NTH, to the total noise power, NT, said total noise power also comprising a shot noise power contribution, NQ, comprising the steps of;

(i) generating a broadband, substantially shot noise limited optical reference signal (21), OPTREF,
(ii) detecting the optical signals, OPTDUT and OPTREF, (3,21) and converting said optical signals into equivalent electrical signals, ELECDUT and ELECREF respectively, using detection means (10), wherein ELECDUT and ELECREF each comprise an ac component and a dc component (13), wherein the dc components of the electrical signals, ELECDUT and ELECREF, are idc(DUT) and idc(REF) respectively, and varying the optical reference signal, OPTREF, (21) so that idc(DUT) and idc(REF) are substantially equal,
(iii) separating the ac component and the dc component (13) of the electrical signals, ELECDUT and ELECREF,
(iv) measuring one or more of the dc components, idc(DUT) and idc(REF), of the electrical signals, ELECDUT and ELECREF,
(vi) amplifying one or more of the ac components of the electrical signals, ELECDUT and ELECREF,
(vii) measuring one or more of the amplified ac components of the electrical signals, ELECDUT and ELECREF,
(viii) measuring the ac component of the electrical signal output from the detection means (10) in the absence of OPTDUT and OPTREF, (3, 21) so as to provide an indication of the thermal noise power, NTH, and
(ix) deriving the device noise power contribution, NL, to the optical output signal, OPTDUT, (3, 21) from the measured values of the ac components and the dc components of the electrical signals, ELECDUT and ELECREF.

5 more elements

Dependencies are inferred from the claim text.

Device noise measurement system

Life of this patent

Device noise measurement system

Life of this patent

Abstract

Claims21