AU682982B2

AU682982B2 - A low noise photon coupled circuit

Info

Publication number: AU682982B2
Application number: AU35144/95A
Authority: AU
Inventors: Wood Nang Cheung; Paul Julian Edwards
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-09-14
Filing date: 1995-09-14
Publication date: 1997-10-23
Anticipated expiration: 2015-09-14
Also published as: AU3514495A

Description

TITLE: "A LOW NOISE PHOTON COUPLED CIRCUIT"

Technical Field

This invention relates to optoelectronic coupling and more particularly to a photon coupled circuit for applications including low noise optocoupling, optical interconnects, current amplifiers, light amplifiers and optical bistable devices. As used herein, the term "photon coupled circuit" refers to a configuration of optoelectronic devices which are photonically and electronically coupled.

Background Art

Many prior art devices achieve a current amplification by the use of active electronic circuits. Such devices are not able to offer a low noise operation and in particular are often limited by the shot noise in the system. Australian patent 648682 describes an optoelectronic device in which the noise is suppressed below the normal shot noise.

Disclosure of Invention

It is an object of this invention to provide a sub-shot noise photon coupled circuit which incorporates feedback to provide high gain or wide bandwidth characteristics.

Accordingly, in one aspect this invention consists in a low noise photon coupled circuit comprising a high quantum efficiency semiconductor light emitter; means to provide a bias current to said light emitter, the light output of said light emitter being modulated by the current flowing therein; a semiconductor light detector optically coupled with high quantum efficiency with said light emitter, means to provide a bias current to said light detector, the current flowing in said light detector being modulated by the light received from said light emitter, the quantum transfer efficiency of said circuit being greater than 0.5, and means to apply a feedback portion of the current flowing in said light detector to reinforce or oppose the current flowing in the light emitter.

In one form of the invention the feedback is positive to reinforce the current flowing in the light emitter and provide current gain exceeding unity. In another form of the invention the feedback is negative to oppose current flowing in the light emitter and provide an increased bandwidth. As used herein, the term quantum transfer efficiency refers to the efficiency of the transfer of the current modulating the light emitter into current flowing in the light detector. It is therefore equal to the current gain of the circuit.

In one form of the invention the light emitter comprises an array of high quantum efficiency laser diodes or light emitting diodes which are modulated by a common current modulator.

In one form of the invention, the product of the number of elements in the array and the value of the quantum coupling efficiency between each element and the light detector is much greater than 1. In this case the quantum partition noise introduced in the coupling between the light emitting array and the detecting system is negligible.

In another form of the invention, employing a single light emitter the quantum transfer efficiency preferably exceeds 0.75 and ideally is as close to unity as possible.

Preferably, the light detector comprises a PIN photo diode.

The high quantum efficiency coupling between the light emitter and the light detector is preferably achieved by close physical positioning as for example, in an optoelectronic integrated circuit (OEI through and selection of high quantum efficiency devices. High efficiency coupling can also involve the use of an optical waveguide or a high efficiency light pipe.

In one form of the invention the current in the light emitter is preferably modulated by the current output of an additional light detector. The device then functions as a light receiver. In an alternative arrangement an input light signal from an external source is received by the light detector and the current flowing in the light detector is modulated by the input light signal.

In another form of the invention the current flowing in the light detector is used to drive individual or arrays of laser diodes or light emitting diodes. The photon coupled circuit of this invention can be used as the basis of an optocoupler, amplifier, optoelectronic oscillator, optoelectronic multivibrator or optoelectronic bistable device.

Brief Description of Drawings One embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Fig. 1 shows a photon coupled current amplifier according to this invention; Fig. 2 shows an AC equivalent circuit of the amplifier shown in Fig. 1; Fig. 3 shows (a) current gain, (b) voltage gain and (c) power gain of the photon coupled amplifier shown in Fig. 1;

Fig. 4 is an AC equivalent circuit of the photon coupled amplifier used for noise calculations; Fig. 5 is a plot of current gain of the photon coupled amplifier with feedback resistance equal to (a) 30, (b) 15 and (c) 7.5 k ohms. Fig. 6 is a plot of the frequency spectra of the photon coupled amplifier output currents of

(a) signal and (b) noise; Fig. 7 shows a plot of a noise figure variation with a source resistance at different quantum efficiencies; Fig. 8 shows a second embodiment of a photon coupled amplifier according to this invention; and

Fig. 9 shows a third embodiment of a photon coupled amplifier according to this invention.

Best Modes for Carrying Out the Invention

Fig. 1 shows the circuit of an optocoupler amplifier. An array of series connected semiconductor junction light emitting diodes (or laser diodes) TDA each of high quantum efficiency is shown modulated by a voltage source v_g with internal impedance R_g . It is connected to the array through a blocking capacitor C_; and the DC bias current for the array is provided through resistance R_t . The light from this array is efficiently collected by an array of PIN diodes OLD which is biased through resistance R₂ . A fraction of the current output from the detector is fed back from point B through th feedback resistance R, and the blocking capacitor C₂ to the input of the light emitting array at point A to provide positive current feedback. The amplified current output is available between points A and B or between B and G. The noise behaviour of the positive feedback amplifier will depend on the quantum efficiency of the optoelectronic conversion and the noise figure is expected to improve as the quantum efficiency is increased. In the following analysis, a LED model which has been found useful for both small signal and noise performance evaluations is used.

Let -V = number of LEDs in series η = differential quantum efficiency per device r_d = differential resistance of the LED r. = bulk resistance of the LED r* = r, + r_d

R_j = bias resistance for the LED array R₂ = includes the bias and load resistance at the output

R_f = feedback resistance (or impedance in general).

The low-frequency small signal equivalent circuit for the system is shown in Fig. 2. It is assumed that the LEDs are identical and that the detector generates an output cuπent given by i₀ - -V ηι₂), where i₂ is the signal current flowing through the LED array. The impedance of the detector is assumed to be high compared with R_f or R With i, equal to the current from the signal source, it can readily be shown that the closed-loop current gain of the amplifier is given by

where A₀ is the open-loop cuπent gain given by

and β is the feedback factor given by

For stability reasons, the amplifier must operate such that the loop gain A₀ β is less than unity. If /?, and R₂ are large compared with the device resistance r^ , the open-loop gain becomes _Vη and the feedback factor simplifies to R₂ / R_f + R₂ ).

The input impedance of the opto-amplifier is given by

The effect of the LED bias resistance R_; on the input resistance is already included in the expression for the current gain.

It can also be shown that the voltage gain of the feedback amplifier is given by

It will be noted that while the current gain is proportional to the number of LEDs in the array, the voltage gain is independent of N. When R_f = ∞, A_v reduces the open-loop value of rR /r_sd . For a given circuit, the voltage gain increases as the LED bias current increases. This is because the device resistance decreases as the bias current goes up. The apparent power gain of the feedback amplifier is given by P₀ IP_t = A_v A_t , where P_j = v_t i, is the input signal power and P₀ = v₀ i₀ is the output signal power from the detector. Fig. 3 shows typical variations of current gain, voltage gain and power gain of the amplifier as a function of the feedback resistance.

At high frequencies, the LED junction can be represented by a parallel RC circuit. The rate of photons produced, n_p , is related to the diode injection current i₂ by

where τ_c is the charge recombination constant and q is the electronic charge. The detector current is given by

where η(ω) = η/(l+jω τ_c). The high-frequency gain of the amplifier can be found using (2), (3) and (5) with η replaced by η(ω). Assuming the LED resistance to be small then β = R₂ /(R_f +R₂ ) and consequently

The 3 dB bandwidth is thus given by (l--Vηβ)/τ_c and the gain-bandwidth product is Nη t_c. This result is consistent with positive feedback, ie, an increase in low-frequency gain is accompanied by a corresponding reduction in bandwidth. While the GB product is independent of feedback, it is proportional to N which can be increased by using more LEDs in the array. If wide bandwidth is of primary concern, negative feedback can be used. To implement negative feedback in the photon coupled amplifier of Fig. 1, the polarities of the detectors are reversed and the negative bias supply (V-) is replaced by a positive bias supply. This causes the detector current (i₀ ) to reverse in direction. The analysis of the negative feedback amplifier follows the same lines as for positive feedback except that i₀ is now given by - Nτ\i₂ , indicating a change of direction of current flow. In comparison with the positive feedback structure, -η is assigned instead of η to equations (l)-(3) to give the current gain of the negative feedback opto-amplifier, ie.

where A₀' is the open-loop current gain given by

and β' is the feedback factor given by

For large values of R_} and R₂ , the open-loop gain reduces to A = NΎ\ while the feedback factor becomes β' = R₂ l(R_{ + R₂ ), which is equal to β in the case of positive feedback. Therefore the current gain is

„

It can readily be shown that the magnitude of the amplifier gain will drop to JVη/(l+iVηβ) while the bandwidth increases to (l+.Vηβ)/t_c, with GB again given by -Vη/τ_c. By stacking a number of high quantum efficiency LEDs in a negative feedback array amplifier, the bandwidth of the amplifier can be made to substantially exceed the intrinsic bandwidth (l t_c) of a single LED.

The noise spectral density of the detector current consists of two components: the transmitted diode array current noise and the photon partition noise. The partition noise arises from the Bernoulli photon deletion process due to nonideal quantum efficiency. The photon current noise, on the other hand, is the sum of all the random events which independently modulate the injection current of the LED. It consists of the thermal noise of the circuit resistances, the short noise in the LEDs and the feedback of any current fluctuations in the detector, including partition noise. A low-frequency noise equivalent circuit of the positive feedback opto-amplifier is shown in Fig. 4. R_m represents the parallel combination of R_g and R, , and the equivalent input voltage is given by \_m = V_{f (} (R_m IR_g ). The RMS shot noise voltage of each LED is represented by v„ = i, r_d where ι. is the RMS shot noise current. The thermal noise voltage associated with each of the resistances R_m , R_f , R₂ and Nr_s has been omitted from the figure for the sake of clarity.

It has been shown that the partition noise is given by

<i_p ³ > = iVη(l -η)<i,²> (13⁾

where q is the electronic charge, I the LED bias current, and <i.²> (=2qϊ) is the mean square shot noise current spectral density. Standard noise analysis of the circuit of Fig. 4 gives the transmitted photon current noise due to thermal and shot noise sources:

<i 2>-.(_ l_)2 (2£)<J »> (14)

° 1--Vηα μ ^*

where

The total output noise current is given by

^«»>⁼ '> ⁺<Η^{2 >} (17)

where

The second term in (17) represents the total effect of the partition noise, ie, it consists of the open-loop partition noise and the returned partition noise through the feedback loop.

Similar circuit analysis yields the output signal current

where V_g is the input signal voltage. The output signal-to-noise ratio is therefore given by

The input signal and noise are defined as the LED modulation currents due to the signal source V^ and the thermal noise of the source resistance R_g , respectively. Thus the input SNR is given by

where r_d = μkT/ql, μ being the ideality factor.

Finally, the noise figure of the feedback amplifier is given by

I/²] (22)

To compare the noise figure of the feedback amplifier with the open-loop case R_t and R are assumed for simplicity to be large compared with Nr^ which leads to α = R₂ /(R, +R₂)

For the open-loop amplifier, R_f » R₂ so that α and 0 and U = 1. When feedback is used R_f « R₂ so that α = 1 and U = l/(l-Nx]). In both cases (22) simplifies to

That is, under ideal bias conditions, the noise figure of the amplifier is independent o positive feedback. This is equivalent to saying that the output SNR of the amplifier is no affected by the applied feedback (as the input SNR may be fixed at a constant).

When η=l, the last term of (24) vanishes and the noise figure decreases as R_g increases When the quantum efficiency is less than unity, the same term will contribute to the rise o -li¬ the noise figure for large source resistance. Differentiating (24) with respect to R_g gives the optimum source resistance for minimum noise figure.

The corresponding minimum noise figure is then given by

F(min)=l+2 £!i ₎₍Ii₊Ji Ji (26) η r_d 2 2

which is independent of the number of light emitters used. In practice, finite bias resistances will increase F(min) slightly.

For the purposes of verification of the operation of the photon coupled amplifier of Figure 1, an photon coupled amplifier based on a Hamamatus type L2656 AlGaAs/GaAs Heterojunction LED was modelled in Pspice using known techniques. The parameters used for the LED were : r_d = 34.4Ω (at 1 mΛ), r, = 3.6 Ω μ = 1.323 and τ_c = 0.297 μs. With V_t = 1 μV, R_g = 160 Ω R₂ = R₂ = 100 kΩ, N = 1 and η = 0.95, the current gain of the amplifier is obtained as shown in Fig. 5. Curves (a), (b) and (c) are the current gains with the feedback resistance equal to 30, 15 and 7.5 kΩ respectively. It is clearly demonstrated that, as positive feedback is increased, the current gain goes up and the bandwidth decreases correspondingly. Fig. 6 shows the frequency spectra of the output signal and noise currents of the amplifier with the same parameters used to obtain curve (b) in Fig. 5. With constant input voltage, the 3-dB bandwidth of the output current spectrum is wider than that of the current gain response because the input current ι_; increases at high frequencies due to the shunting effect of the LED capacitance. The output SNR at low frequencies can be obtained directly from the spectral plots which is 54.25 dB and the input SNR obtained from (21) is 55.77 dB, thus giving a noise figure of 1.52 dB which agrees with calculation by (22). Fig. 7 shows the noise figure of the positive feedback amplifier with a single LED in the array (-V=l). The results are calculated as a function of the source resistance using the general expression (22) with R_t =R₂ =100 kΩ. Curve (a) is for the ideal case of η=l. The noise figure decreases monotonically to 0 dB as the source resistance increases. This is due to high impedance suppression of the shot noise in the LED. Approximately the same noise figure is obtained when the feedback factor is varied between 0 and 1.

Curves (b) and (c) are the noise figures obtained for η=0.95 and 0.9, respectively. In both cases, increasing the source resistance no longer reduces the amplifier noise figure monotonically. As the source resistance increases, the short noise is suppressed as before but the partition noise which increases as η decreases, becomes dominant hence causing an increase in noise figure. As the source resistance becomes very small, the shot noise current through the LED gradually increases, again causing an increase in noise figure. A minimum noise figure occurs at some intermediate value of the source resistance. For example in curve (b), the minimum noise figure is 1.15 dB and the optimum value of R_g is 160Ω which agrees with calculation using (25). Fig. 7 also shows that the noise figure of the feedback amplifier improves as η increases. This is obviously due to the reduction of partition noise in the amplifier at high quantum transfer efficiency.

Referring to Fig. 8, the current modulator formed by V_g and R₍ in Fig. 1 is replaced by an input light detecting system, ILD. The current generated by ILD when light falls on it is used to modulate the array of light emitters. This current is amplified in the same way as in the form of the invention shown in Fig. 1, with the TDA bias and OLD bias currents provided in the same way. Resistance R3 and voltage supply V_b provide DC bias current for the output light emitting array, OTDA.

The output current from OLD is used to modulate the current in the laser diode or light emitting array, OTDA. Capacitor C3 couples the signal from OLD to OTDA. Impedances

Z, and Z, together determine the feedback factor β. The circuit functions as a low noise light amplifier. Another embodiment of the invention is shown in Fig 9. In Fig 9 the LED array labelled TDA shown in Fig. 1 consists of a single light emitting diode or laser diode and the detecting system consists of a single detector.

Providing the quantum transfer efficiency, otherwise called the open-loop current gain, between the light emitting element and the detector is greater than one half, the closed- loop current gain of this configuration can exceed unity. In this form of the invention a current gain exceeding unity can thus be obtained by applying positive current feedback from a single detector to a single emitter.

Referring to Fig. 9, voltage source v_s , with resistance R_s, generates current i, to drive the light emitter LE through coupling capacitor C,. The DC bias current for the semiconductor light emitting junction is provided through resistance R, from the positive voltage power supply V+. Semiconductor light detector LD is optically coupled to light emitter LE to provide open-loop current transfer ratio (η = i_a li_d ) of at least one half. DC bias current for the semiconductor junction diode detector is provided through resistance R₂ from power supply V-. Positive current feedback is provided via capacitor C_f. The output signal may be measured in the form of a voltage drop across impedance Z₀.

The use of high impedances for R₂ , R, and Z. is necessary to obtain large suppression of the diode shot noise cuπent which may otherwise flow through the light emitting element and be amplified. Almost all the output cuπent i_B then flows back via C_f through the light emitter LE and the closed loop cuπent gain is then approximately equal to η /(1-η).

Photon partition noise is minimised by aπanging the open-loop cuπent transfer ratio, otherwise called the quantum transfer efficiency, to be as close to unity as possible.

The circuit of Fig. 9 can also be used as the basis of a lightwave amplifier and receiver by inputting light directly to detector LD.

It will be apparent that the best performance of the photon coupled circuit according to this invention is achieved using a single mode laser diode, resonant cavity LED, or micro cavity laser diode or LED with the highest possible quantum efficiency. Cuπently available LEDs have an external efficiency at room temperature in excess of 85%.

Claims

1. A low noise photon coupled circuit comprising a high quantum efficiency semiconductor light emitter, means to provide a bias current to said light emitter, the light output of said light emitter being modulated by the current flowing therein; a semiconductor light detector optically coupled with high quantum efficiency with said light emitter; means to provide a bias cuπent to said light detector, the cuπent flowing in said light detector being modulated by the light received from said light emitter, the quantum transfer efficiency of said circuit being greater than 0.5, and means to apply a feedback portion of the cuπent flowing in said light detector to reinforce or oppose the cuπent flowing in the light emitter.

2. A photon coupled circuit as claimed in claim 1 wherein said feedback is positive to reinforce the cuπent flowing in the light emitter and provide increased gain.

3. A photon coupled circuit as claimed in claim 1 wherein said feedback is negative to oppose the cuπent flowing in the light emitter and provide an increased bandwidth.

4. A photon coupled circuit as claimed in any one of claims 1 to 3 wherein said light emitter comprises an aπay of semiconductor light emitters.

5. A photon coupled circuit as claimed in claim 4 wherein said aπay comprises a plurality of laser diodes or light emitting diodes connected in series, and modulated by a common cuπent modulator.

6. A photon coupled circuit as claimed in claim 4 or claim 5 wherein the product of the number of elements in the aπay and the value of the quantum coupling efficiency between each element and the light detector is much greater than 1.

7. A photon coupled circuit as claimed in any one of claims 1 to 3, wherein said light emitter is a single semiconductor device and said light detector is a single semiconductor device.

8. A photon coupled circuit as claimed in claim 7, wherein said semiconductor device comprising the light emitter is a single light emitting diode or laser diode.

9. A photon coupled circuit as claimed in any one of claims 1 to 8 wherein the light detector comprises a photodiode.

10. A photon coupled circuit as claimed in any one of claims 1 to 9 wherein the high quantum efficiency coupling of said light emitter with said light detector is achieved by close physical positioning.

11. A photon coupled circuit as claimed in any one of claims 1 to 9, wherein the high quantum efficiency coupling of said light emitter with said light detector is achieved via an optical waveguide or high efficiency light pipe.

12. A photon coupled circuit as claimed in claim 10 or claim 11 wherein said circuit is in the form of an optoelectronic integrated circuit.

13. A photon coupled circuit as claimed in any one of claims 1 to 12 wherein the quantum transfer efficiency of said coupling is at least 75%.

14. A photon coupled circuit as claimed in any one of claims 1 to 13 further comprising a second semiconductor light detector, the output cuπent of said second light detector modulating the cuπent flowing in said light emitter.

15. A photon coupled circuit as claimed in any one of claims 1 to 13 wherein said light detector is adapted to receive an input light signal from an external source and the current flowing in the light detector is modulated by the input light signal.

16. A photon coupled circuit as claimed in claim 15 further comprising a light output device driven by the amplified cuπent from said light detector.

17. A photon coupled circuit as claimed in claim 16 wherein said light output device comprises one or more laser diodes or light emitting diodes.

18. A photon coupled circuit substantially, as described with reference to Figures 1 to 7, Figure 8 or Figure 9 of the accompanying drawings.