Method and apparatus for optical frequency modulation characterization of laser sources

The present invention provides a cost-effective method and apparatus for optical frequency modulation (FM) characterization of a laser source and provide a measure of laser chirp by using inexpensive equipment that is readily available. In order to measure the chirp, the invention makes use of interferometry without resorting to expensive and bulky equipment such as optical network or optical spectrum analyzers. In a preferred embodiment, the chirp characteristic of a laser source is measured by injecting an input signal at a particular modulation frequency into the laser source under test to generate an optical signal and passing the optical 15 signal generated into an asymmetrical Mach Zehnder interferometer (AMZI). Because the AMZI exhibits a cyclical wavelength to photo-current response, a number of high frequency ripples or interference ripples are introduced in the optical signal. According to the invention, the interference ripples introduced within a half modulation period of the laser input signal are counted to provide a measure of the approximate chirp amount on the modulated frequency.

FIELD OF THE INVENTION
 The present invention relates generally to the characterization of
 communication laser sources, and more particularly to the optical
 frequency modulation characterization of communication laser sources by
 interferometry.
 BACKGROUND OF THE INVENTION
 In a typical fiber optic transmission network, optical frequency modulation
 (FM) characteristics of communication laser sources such as laser diodes
 must be accurately determined in order to define the limits within which
 information can be reliably modulated and exchanged between optical
 terminals in the network. The optical frequency behavior of a particular
 laser source under modulation is determinative of the ability of that
 laser source to transmit information. When a laser source is directly
 modulated with an input current signal, the laser source output exhibits a
 frequency deviation (.DELTA.f) per milliAmpere which varies as a function
 of the laser input modulation frequency. This FM characteristic is
 commonly referred to as the laser chirp. For most optical systems, an
 accurate characterization of laser chirping is essential to ensure
 transmission accuracy and improve performance. For example, in wavelength
 division multiplex (WDM) networks, laser chirp measurements are used to
 fine tune the laser operating wavelengths for an optimal use of the
 available optical bandwidth thereby enhancing transmission performance.
 Presently, laser chirp is measured with specialized equipment that
 typically includes sophisticated network or optical spectrum analyzers.
 For network applications using multiple laser sources, the use of this
 equipment is not cost-effective. In a typical WDM network for example,
 many laser sources are used in different transmitting terminals which
 necessitates the duplication of the chirp measuring equipment used for
 each transmitting location. Duplicating this equipment for each
 transmitting location in the network may very rapidly prove to have a
 major impact on the cost associated with chirp characterization.
 There may be situations where it is practical to use the same equipment for
 laser sources in different locations. In these cases, it would also be
 desirable to have chirp-measuring equipment that is simple and more
 portable. This is due to the fact that the chirp measurement equipment
 currently used is heavy and bulky which makes it difficult to transport
 between transmitting sites.
 SUMMARY OF THE INVENTION
 The present invention addresses these issues and to this end provides a
 methodology and apparatus to avoid the present limitations in this art.
 The present invention provides a cost-effective apparatus and method for
 optical frequency modulation (FM) characterization of a laser source and
 provide a measure of laser chirp by using inexpensive equipment that is
 readily available. The invention makes use of interferometry to
 efficiently measure the chirp of a laser source in a simple manner without
 resorting to expensive and bulky equipment such as optical network or
 optical spectrum analyzers.
 According to a preferred embodiment, the chirp characteristic of a laser
 source can be measured by injecting an input signal at various modulation
 frequencies into the laser source under test and passing the optical
 signal generated by the laser source into an asymmetrical Mach Zehnder
 interferometer (AMZI). For each modulation frequency used, the laser input
 signal induces an optical frequency deviation in he optical signal
 generated which is converted by the AMZI into cyclical variations in
 optical power of the optical signal. Because the AMZI exhibits a cyclical
 wavelength to photo-current response, a number of high frequency ripples
 (hereinafter referred to as interfering ripples) are introduced in the
 optical signal. According to the invention, the interference ripples
 introduced therein are counted over a half modulation period of the laser
 input signal to provide a measure of the approximate chirp amount on each
 modulated frequency.
 In order to determine the number of interfering ripples brought about by
 the AMZI, the optical signal is converted into an electrical form which is
 representative of the interfering ripples introduced. The electrical
 signal is then subsequently applied to an oscilloscope for an estimation
 of the number of ripples within a half modulation period of the laser
 input signal.
 For optimal chirp measurement, the AMZI is preferably designed with a short
 free spectral range (FSR) so as to introduce a large number of
 interference ripples within a half modulation period. According to the
 invention, the AMZI FSR is used as a measuring unit and defines the degree
 of accuracy with which chirp measurements are made.
 Advantageously, the chirp measurement apparatus used by the present
 invention is simple and more portable. In particular, the interferometry
 circuit used for converting variations in frequency in the laser output
 signal into variations in optical power can be easily transported or
 alternatively duplicated and integrated in multiple transmitting sites at
 a reasonable cost. Further, the invention advantageously uses a low-cost
 oscilloscope which is also more portable than the optical network or
 optical spectrum analyzers used in conventional chirp measurement systems.
 Other aspects and features of the present invention will become apparent to
 those ordinarily skilled in the art upon review of the following
 description of specific embodiments of the invention in conjunction with
 the accompanying figures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 As is well known, conventional laser sources such as laser diodes exhibit
 modulation characteristics which make them well-suited for transmitting
 information optically. As shown in FIGS. 1A and 1B, varying the laser
 input current induces variations in the optical frequency (frequency
 modulation) and power (intensity modulation) of the laser output signal
 generated. In particular, FIG. 1A shows as a example, an optical frequency
 plot of the output of a typical laser source denoted by f(i.sub.LD) as a
 function of laser input current i.sub.LD for a low frequency laser input
 current modulation regime. The frequency response of the laser source is
 dependent upon the laser input current modulation frequency and the
 frequency plot f(i.sub.LD) illustrated in Figure is only representative of
 a low frequency modulation regime. FIG. 1B shows also as a example, an
 optical power plot p(i.sub.LD) of the same laser output also as a function
 of the laser input current i.sub.LD.
 In order to properly use a laser source to optically transmit information,
 an optical frequency modulation (FM) characterization of the laser source
 must be carried-out in order to define the limits within which information
 can be reliably transmitted and exchanged between optical terminals in the
 network. This optical FM characterization includes a determination of what
 is generally known as laser chirp. Chirp in a laser source is
 determinative of the ability of that particular laser source to transmit
 information and is the result of a number of transient effects which occur
 as a result of changes in the laser input modulation frequency. For a
 particular modulation signal current and frequency, laser chirp is defined
 as the frequency deviation (.DELTA.f) observed in the laser source output
 signal per milliampere of laser input current as a function of the laser
 input modulation frequency.
 To further illustrate this, reference is now made to FIG. 2 where a plot of
 the laser chirp or optical FM deviation transfer curve of a typical DFB
 laser diode is shown for a modulation frequency range extending from 0 Hz
 to 1 GHz. From this figure, it can be observed that the frequency shift of
 the DFB laser diode is approximately 0.5 GHz per mA for modulation
 frequencies up to around 10 MHz. This shift is principally caused by
 thermal chirp. For modulation frequencies over 10 MHz up to 1 GHz, it can
 be observed that there is a gradual roll-off of the amplitude of the laser
 diode output frequency swing. In this frequency range, the thermal effects
 decrease and the frequency swing is increasingly caused by other
 characteristics of the laser diode such as adiabatic chirp and relaxation
 oscillations. These characteristics are well-known in the art and, as
 such, are not discussed here in any detail.
 The present invention provides a cost-effective method and apparatus for
 measuring chirp in laser sources. The invention makes use of asymmetrical
 Mach-Zehnder interferometry to efficiently measure laser chirp without
 resorting to expensive and bulky equipment such as optical network or
 optical spectrum analyzers. The invention can be used to measure the chirp
 characteristic of any laser source. For clarity however, the invention
 will now be described only in relation to laser diodes.
 Referring now to FIG. 3, there is illustrated a preferred embodiment of the
 invention for measuring the chirp characteristic of a particular laser
 diode 2. This invention makes use of an AMZI generally indicated by 10 and
 a balanced photo-detector generally indicated by 18. The balanced detector
 18 consists of a pair of photo-diodes 14, 16 connected in a push-pull or
 balanced detection arrangement. The detector 18 is connected to an
 oscilloscope 20 for determining the chirp of the laser diode 2 under test.
 In order to measure the chirp of the laser diode 2, an input signal
 i.sub.LD (t) of a particular modulation frequency f.sub.m is injected into
 the laser diode 2. In the preferred embodiment of FIG. 3, the input signal
 i.sub.LD (t) is shown as generated by a signal generator 22 connected to
 the laser diode 2 under test. It is understood that the input signal
 i.sub.LD (t) could alternatively be generated by other signal generating
 means such as, for example, a signal generating function embodied in the
 oscilloscope 20.
 In response to the input signal i.sub.LD (t) applied, the laser diode 2
 generates an optical signal o(t) whose frequency and optical power
 characteristics are reflective of the input signal current. According to
 the invention, the chirp produced by the laser diode 2 for the particular
 modulation frequency of the input signal current can be measured by
 passing the optical signal o(t) generated into an asymmetrical
 Mach-Zehnder interferometer (AMZI) and counting the number of interfering
 ripples introduced by the AMZI within a half modulation period of the
 laser input signal i.sub.LD (t). As will be described below in further
 detail, the number of interfering ripples counted provides a measure of
 the approximate chirp amount exhibited by the laser diode 2 for a
 particular laser input signal modulation frequency.
 The laser input signal i.sub.LD (t) necessary for measuring the chirp of
 the laser diode 2 under test can take various forms provided it is
 cyclical with amplitude levels all above the lasing threshold such that
 the laser diode can operate in a continuous mode. Preferably, the input
 signal i.sub.LD (t) is sinusoidal (or triangular). For the purpose of
 example, FIGS. 4A and 4B show referenced the optical frequency and power
 characteristics curves of FIGS. 1A and 1B a sinusoidal laser input signal
 i.sub.sin (t) at an operating point A which can be used to determine the
 chirp of the laser diode 2 under test.
 At the operating point A, the laser input signal i.sub.sin (t) has an
 average DC value denoted by I.sub.DC around which it oscillates at a
 modulation frequency f.sub.m with a peak-to-peak amplitude .DELTA.I. The
 operating point A is selected to confine the peak-to-peak excursions of
 the laser input signal i.sub.sin (t) above the lasing threshold such that
 the laser diode 2 always operates in a linear region.
 FIGS. 4A and 4B also show the optical frequency and power characteristics
 of a corresponding optical signal o.sub.sin (t) generated by the laser
 diode 2 as a result of direct modulation by the laser input signal
 i.sub.sin (t). The optical signal o.sub.sin (t) is shown in the form of
 its optical frequency and power characteristics. FIG. 4A shows referenced
 to the optical frequency/time plot of FIG. 1A, an optical frequency/time
 plot f.sub.sin (t) of the laser diode output signal o.sub.sin (t) at the
 operating point A and FIG. 4B shows referenced to the optical power/time
 plot of FIG. 1B, an optical power/time plot p.sub.sin (t) of the laser
 diode output signal o.sub.sin (t) at the same operating point A.
 From these figures, it can be observed that in response to this sinusoidal
 current modulation by i.sub.sin (t), the laser diode output power
 p.sub.sin (t) varies sinusoidally at the modulation frequency f.sub.m with
 a peak-to-peak amplitude of .DELTA.P. It can also be observed that the
 laser input signal i.sub.sin (t) also induces in the laser output signal
 o.sub.sin (t) a sinusoidal optical frequency deviation denoted by
 f.sub.sin (t) at the modulation frequency f.sub.m with a peak-to-peak
 amplitude .DELTA.f. The optical frequency deviation .DELTA.f results in a
 laser chirp (.DELTA.f/mA) which, as noted before, is expressed as a
 function of the modulation frequency f.sub.m.
 Referring back to FIG. 3, in order to measure the laser chirp produced by
 the laser diode 2 under test, the frequency deviation .DELTA.f occurring
 in the laser output signal o.sub.sin (t) must be determined. For this, the
 optical signal o.sub.sin (t) is fed into the AMZI 10 which operates to
 convert the optical frequency variations in the optical signal o.sub.sin
 (t) to cyclical variations in optical power. As will be explained below in
 further detail, the AMZI 10 exhibits a wavelength to photo-current
 response formed of a repetition of peaks and valleys which introduces high
 frequency ripples (hereinafter also referred to as interference ripples)
 in the laser output signal power p.sub.sin (t).
 Referring back to FIG. 3, the AMZI 10 has two inputs 30, 31 and two outputs
 9, 11. The AMZI 10 is designed to receive the laser output signal
 o.sub.sin (t) in one input 30 while the other input 31 is left
 unconnected. The AMZI 10 consists of two 3dB fiber couplers 6, 8
 interconnected with two single-mode fiber arms 5, 7 and a delay line 12 of
 a variable length L located on arm 7. The AMZI outputs 9, 11 produce an
 intermediate optical signal in the form of complementary output signals
 which are denoted by s.sub.out1 (t) and s.sub.out2 (t). The AMZI outputs
 9, 11 are coupled respectively to the photo-diodes 14 and 16 of the
 balanced photo-detector 18. The photo-detector 18 produces an electrical
 output signal v.sub.sin (t) which is fed on line 28 into an oscilloscope
 20 for chirp measurement.
 In the AMZI 10, the coupler 6 functions as a splitter, dividing the
 o.sub.sin (t) optical signal power equally to form two optical signals,
 s.sub.1 (t), s.sub.2 (t) which respectively travel in the fiber arms 5 and
 7. By operation of the delay line 12, the optical signal s.sub.2 (t)
 propagating in arm 7 is delayed by a delay .tau. such that at the coupler
 8, it interferes with the other optical signal s.sub.1 (t) traveling in
 arm 5 to produce the complementary signals s.sub.out1 (t) at output 9 and
 s.sub.out2 (t) at output 11. The AMZI outputs 9, 11 are each characterized
 by a respective power transfer function H.sub.1 (f), H.sub.2 (f) which are
 known to be:
EQU H.sub.1 (f)=A+Bsin.sup.2 (.pi.f* .tau.), H.sub.2 (f)=A+Bcos.sup.2
 (.pi.*f*.pi.)
 where A and B are scaling factors.
 FIG. 5 shows a frequency plot of each AMZI output power transfer function
 H.sub.1 (f), H.sub.2 (f) which are respectively drawn as a full and a
 dotted line. The period of free spectral range (FSR) for each AMZI outputs
 9 and 11 is .tau..sup.-1 and their respective power transfer functions
 H.sub.1 (f), H.sub.2 (f) are sinusoidal and out of phase by 180 degrees.
 The AMZI output power transfer functions H.sub.1 (f), H.sub.2 (f) are
 indicative of the amount of optical power generated at each AMZI output 9,
 11 as a function of the operative frequency of the laser output signal
 o.sub.sin (t). In terms of optical power produced, the AMZI outputs 9, 11
 are complementary to one another such that for any given o.sub.sin (t)
 frequency, the sum of the optical power produced by each AMZI output 9, 11
 is always equal to the amount of optical power carried by o.sub.sin (t) at
 that particular frequency.
 The present invention takes advantage of the cyclical nature of the AMZI
 frequency response to provide an effective measurement mechanism for
 determining chirp in the laser diode 2 for a particular modulation
 frequency f.sub.m. As noted above, when the laser input signal i.sub.sin
 (t) induces a frequency deviation .DELTA.f in the laser output signal
 o.sub.sin (t), the AMZI 10 introduces a number of interfering ripples in
 the laser output signal o.sub.sin (t) such that the optical frequency
 variation induced therein is converted into cyclical optical power
 variations or interference ripples.
 Each interference ripple introduced in o.sub.sin (t) corresponds to a full
 cycle of the AMZI frequency response (see FIG. 5) and denotes a frequency
 shift in the optical signal o.sub.sin (t) equal to the AMZI frequency
 cycle bandwidth or FSR (1/.tau.). According to the present invention, the
 AMZI FSR is used as a measuring unit for determining the frequency
 deviation .DELTA.f and defines the degree of accuracy with which the
 frequency deviation is measured.
 For optimal chirp measurement, the AMZI 10 is 20 designed with a short
 frequency spectral repetition by selecting an appropriate FSR that can
 provide a sufficiently accurate measure of the frequency deviation
 .DELTA.f. Generally, the FSR is selected in relation to the particular
 laser input signal modulation frequencies used for measuring the laser
 chirp. As an example, for a laser input signal i.sub.sin (t) with a
 modulation frequency less than 10 MHz, an FSR of 1 GHz would be sufficient
 to provide a reasonably accurate measure of the frequency deviation
 .DELTA.f. For modulation frequencies greater than 10 MHz, an FSR of 1 GHz
 would still be adequate although a smaller FSR (larger delay .tau.) could
 be used to provide a more accurate measure of the frequency deviation
 .DELTA.f.
 Once selected to provide an adequate measuring unit for determining the
 frequency deviation .DELTA.f with acceptable accuracy, the AMZI FSR
 (1/.tau.) is implemented by selecting an appropriate length L for the
 delay line 12. With the FSR (1/.tau.) determined, the corresponding delay
 .tau. can be identified since, as noted above, an FSR period is equal to
 1/.tau.. If the effective refractive index .eta..sub.eff of the AMZI fiber
 arms 16, 18 is known, the desired fiber length, L, for a time delay .tau.
 can be determined using the following equation:
EQU L=(c/.eta..sub.eff)*.tau.
 where c is the speed of light in vacuum. As this delay line 12 is a
 function of the AMZI FSR, it ultimately dictates the accuracy level with
 which the frequency deviation induced in the optical signal o.sub.sin (t)
 is calculated.
 As an example, if an FSR of 1 GHZ is selected, the necessary length for the
 delay line 12 can be determined by first calculating the delay .tau.
 corresponding to a 1 GHz FSR. With the FSR known, the corresponding delay
 .tau. can be identified with the above expression:
EQU FSR=1/.tau.=1 GHz;
 From this, the necessary delay .tau. for the delay line 12 (see FIG. 5A)
 can be calculated as follows:
EQU .tau.=1/1 GHz=1 nS
 According to the above, .tau.=1 nS. If the effective refractive index of
 the fiber arms 16, 18 is known to be .eta..sub.eff =2, the desired fiber
 length, L, for a time delay .tau.=1 nS can be determined with the above
 equation as follows:
 2, the desired fiber length, L, for a time delay .tau.=1 nS can be
 determined with the above equation as follows:
EQU L=(c/.eta..sub.eff)*.tau.=(3*10.sup.8 m/S)/2*1 nS=0.2 meter
 With this delay line length, the AMZI will exhibit a 1 GHz FSR. As will be
 explained below in further details, the AMZI FSR as selected can then be
 used to determine the frequency variation .DELTA.f which in turn, can be
 used to establish the laser chirp of the laser diode 2 exhibited under
 test at the modulation frequency f.sub.m.
 The number of ripples introduced is directly proportional to the size of
 the frequency deviation .DELTA.f induced in the laser output signal
 o.sub.sin (t). More specifically, the larger the frequency deviation
 .DELTA.f, the greater the number of interfering ripples introduced.
 Conversely, the smaller the frequency deviation .DELTA.f, the smaller the
 number of interfering ripples introduced. According to the invention, the
 ripples introduced within a half period of the modulation frequency
 f.sub.m are counted to provide a measure of the frequency deviation
 .DELTA.f for the particular modulation frequency f.sub.m at which the
 laser input signal i.sub.sin (t) operates. By operating i.sub.sin (t) at
 different modulation frequencies and counting the number of interfering
 ripples produced on each modulated frequency, a chirp characteristic curve
 such as that illustrated in FIG. 2 can be obtained which shows how the
 chirp for the particular laser diode 2 under test varies as a function of
 the laser input modulation frequency.
 In order to determine the number of interfering optical power ripples
 brought about by the AMZI 10, the AMZI output signals s.sub.out1 (t),
 s.sub.out2 (t) are converted into the electrical signal v.sub.sin (t) by
 the balanced detector 18. By converting the AMZI output signals s.sub.out1
 (t), s.sub.out2 (t) to an electrical form, the optical power variations
 are proportionally translated into voltage ripples where each voltage
 ripple corresponds to a respective interference ripple introduced by the
 AMZI 10. These voltages ripples can then be counted with the oscilloscope
 20 for measuring the frequency deviation and hence the chirp exhibited by
 the laser diode 20 under test at the particular modulation frequency
 f.sub.m.
 To further illustrate this, FIG. 6 shows a magnified plot of the sinusoidal
 signal current i.sub.sin (t) shown in FIGS. 4A and 4B and the resulting
 electrical signal i.sub.sin (t) produced by the balanced detector 18. The
 signal v.sub.sin (t) is shown as oscillating around zero between positive
 and negative voltage amplitudes respectively denoted by +V and -V as a
 result of the balanced detection. For clarity, i.sub.sin (t) and v.sub.sin
 (t) are respectively drawn as a dotted and a full line.
 In this particular sinusoidal laser input signal i.sub.sin (t) example, the
 frequency deviation .DELTA.f induced in the laser output signal o.sub.sin
 (t) occurs during a peak-to-peak excursion of the laser input signal
 i.sub.sin (t) over a half modulation period 1/2f.sub.m or t.sub.m /2. When
 the laser output signal o.sub.sin (t) undergoes a frequency deviation
 .DELTA.f, the output signal o.sub.sin (t) sweeps across a number of AMZI
 frequency response cycles (see FIG. 5). As the laser input signal
 i.sub.sin (t) varies sinusoidally at a constant modulation frequency
 f.sub.m and peak-to-peak amplitude, the output signal o.sub.sin (t) will
 continually sweep across the same number of AMZI frequency response cycles
 back and forth at the modulation frequency f.sub.m.
 As a result of the sweep, the AMZI introduces interference ripples or
 cyclical variations in the optical power p.sub.sin (t) of the optical
 signal o.sub.sin (t) where each interference ripple corresponds to a cycle
 of the AMZI frequency response. The interference ripples induced are
 proportionally translated by the balanced detector 18 into voltage ripples
 in v.sub.sin (t) where each voltage ripple corresponds to a respective
 interference ripple introduced by the AMZI 10. Within a half modulation
 period (1/2f.sub.m) of the electrical signal v.sub.sin (t), there are M
 interference ripples introduced in the laser output signal power p.sub.sin
 (t). From FIG. 6, it can be observed that in an electrical form, these
 interference ripples appear as M voltage ripples in the electrical signal
 v.sub.sin (t).
 In accordance with the present invention, the voltage ripples introduced
 within a half modulation period (1/2f.sub.m) of the laser input signal,
 such as, for example, the sinusoidal laser input signal i.sub.sin (t), are
 counted with the oscilloscope 20 to determine the frequency deviation
 .DELTA.f and calculate the associated laser chirp for the particular
 modulation frequency f.sub.m at which the laser input signal operates.
 There are various ways of counting the number of ripples with the
 oscilloscope 20. Preferably, the ripples are counted by a user visually
 inspecting the electrical signal v.sub.sin (t) after it is displayed on
 the oscilloscope 20. For more accuracy, the ripples could alternatively be
 counted by a counting function embodied in the oscilloscope 20 (if
 available).
 Depending on the laser input signal i.sub.sin (t) and hence the frequency
 deviation .DELTA.f induced in the laser output signal o.sub.sin (t), the
 number of interference ripples introduced by the AMZI 10 may not
 necessarily correspond to an integer value. In addition to a sweep of
 multiple AMZI frequency response cycles, there will be situations where
 the frequency deviation .DELTA.f causes the laser output signal o.sub.sin
 (t) to sweep across an additional fraction of a cycle of the AMZI
 frequency response. As it can be observed in FIG. 6, this would occur at
 the peaks and valleys of the laser input current i.sub.sin (t).
 More specifically, during a half modulation period (1/2f.sub.m), the laser
 input current i.sub.sin (t) is increased (or decreased) such that it
 induces a frequency deviation .DELTA.f in the laser output signal
 o.sub.sin (t) which, as a result, sweeps across a number M of AMZI
 frequency response cycles in a particular direction along the AMZI
 frequency response. When the laser input current i.sub.sin (t) is at an
 amplitude peak (or valley), the laser output signal o.sub.sin (t)
 completes its sweep of the AMZI frequency response in that particular
 direction and may finish the sweep anywhere within a particular AMZI
 frequency response cycle which, as a result, is only partially swept.
 As the laser input current i.sub.sin (t) is reduced (or increased), the
 laser output signal o.sub.sin (t) initiates a new sweep in an opposite
 direction along the AMZI frequency response, sweeping initially across the
 same partially swept cycle and subsequently across M other cycles until
 the laser input current i.sub.sin (t) reaches the next valley (or peak) at
 which time the laser output signal o.sub.sin (t) completes its sweep of
 the AMZI frequency response in that particular direction and again, may
 finish the sweep anywhere within a particular AMZI frequency response
 cycle. This would also result in a partial sweep of this cycle.
 As a result of these partial sweeps, the AMZI introduces interference
 ripples or cyclical variations which are not as prominent as the
 interference ripples introduced as a result of a complete sweep.
 Nevertheless, these interference ripples are also proportionally
 translated by the balanced detector 18 into voltage ripples in v.sub.sin
 (t). The voltage ripples introduced as a result of a partial sweep do not
 follow the laser input signal envelope and, as a result, are shown in FIG.
 6 with a lower amplitude.
 As a result of these partial sweeps, the count accuracy for the ripples
 counted within a half modulation period (1/2f.sub.m) of the laser input
 signal i.sub.sin (t) is given by .+-.1/M where M is the total number of
 ripples counted within a half modulation period (1/2f.sub.m). For this, it
 follows that the count accuracy can be improved by selecting a small
 enough FSR relative to the modulation period tm such that the number of
 ripples M present within a half modulation period (1/2f.sub.m) is
 sufficiently large.
 Once the number of voltage ripples present in a half modulation period
 (1/2f.sub.m) is counted, the frequency deviation .DELTA.f can be
 determined by multiplying the voltage ripple count by the AMZI FSR. If M
 ripples are counted within a half modulation period (1/2f.sub.m), the
 frequency deviation is given by:
EQU .DELTA.f=M*(1/.tau.)
 with a frequency deviation accuracy of .+-.1/M*(1/.tau.).
 Once the frequency deviation .DELTA.f present on the modulation frequency
 f.sub.m is determined, the laser chirp on that particular modulation
 frequency can be calculated since, as noted above, the chirp is the
 frequency deviation (peak-to-peak) per mA of laser input signal current:
EQU chirp(f.sub.m)=.DELTA.f per mA
 As an example, for a 1 mA peak-to-peak laser input signal i.sub.sin (t)
 inducing, for example, a frequency deviation of .DELTA.f=500 MHz, the
 laser chirp is calculated as follows:
EQU chirp(f.sub.m)=.DELTA.f/1 mA=500 MHz per mA
 As noted above, by operating i.sub.sin (t) at different modulation
 frequencies and counting the number of interfering ripples produced on
 each modulated frequency, a chirp characteristic curve such as that
 illustrated in FIG. 2 can be obtained which shows how the chirp for the
 particular laser diode 2 under test varies as a function of the laser
 input modulation frequency.
 As an example, FIG. 7 illustrates a laser chirp characteristic curve
 obtained by plotting on a logarithmic scale frequency deviation
 measurements of a typical distributed feedback (DFB) laser diode for the
 sinusoidal laser input signal current i.sub.sin (t) operated in a
 modulation frequency range extending from 0 Hz to 3 GHz. From this figure,
 it can be observed that the DFB laser diode chirp starts at approximately
 500 MHz per mA at a modulation frequency of 100 Hz. As the modulation
 frequency increases, the chirp gradually decreases (as shown in FIG. 2) to
 eventually fall to approximately 2 MHz per mA for a modulation frequency
 of 3 GHz. In this particular example, the chirp measurement resolution for
 modulation frequencies between 0 Hz to 1 MHz is 1.43/10 GHz and the
 resolution for modulation frequencies between 1 MHz and 10 GHz is 0.2/40
 GHz.
 Although embodiments of the invention have been described above, it is not
 limited thereto and it will be apparent to those skilled in the art that
 numerous modifications form part of the present invention insofar as they
 do not depart from the spirit, nature and scope of the claimed and
 described invention.
 The AMZI has been described above as converting optical frequency
 variations in an optical signal into cyclical power variations for chirp
 measurement. It is to be understood that other interferometers converting
 variations of optical frequency in cyclical variations in optical power
 could be used. In particular, any other optical device exhibiting a
 cyclical transfer function to convert an optical frequency variation into
 cyclical variations in optical power can be used in accordance with the
 principles described therein for chirp measurement.
 The AMZI has been described above as implemented with two fiber couplers
 (alternatively referred to as splitters) interconnected with two
 single-mode fiber arms and a delay line. It is to be understood that other
 implementations could be used. For example, the AMZI could alternatively
 be implemented with fused bionic tapered filters or planar light wave
 circuitry and still provide the necessary cyclical frequency-to-power
 conversion in accordance with the present invention.
 The present invention is not restricted to the balanced photo-detector
 described therein. Any other photo-detector capable of converting an
 optical signal into an electrical signal could also be used. For example,
 a single photo-detector connected to either one of the AMZI outputs could
 alternatively be used. In this case, the amount of optical power converted
 by a single photo-detector would always be half that converted by a
 balanced arrangement and the resulting electrical signal v.sub.sin (t)
 produced would be either positive, oscillating between 0V and +V or
 negative, oscillating between 0V and -V, depending on which AMZI output is
 used and whether the photo-detector is pulled to a negative voltage source
 or a positive voltage source. This is in contrast to an electrical signal
 produced by a balanced arrangement as was described above which instead
 would be oscillating around 0V between positive and negative voltage
 amplitudes +V and -V.