Photon counting communication adaptive receiving apparatus and method

Abstract

The application relates to the technical field of wireless optical communication, and discloses a kind of photonic counting communication adaptive receiving device and method, device includes half-wave plate, polarization beam splitter, quarter-wave plate, corner reflector, beam splitter, area array detector and single-photon detector;Method includes: incident light passes through half-wave plate, polarization beam splitter, quarter-wave plate in turn to reach corner reflector, is reflected again and then passes through quarter-wave plate into polarization beam splitter, is reflected and then enters beam splitter;Beam splitter divides incident light into two ways, area array detector measures the exit energy of one-way light, adjusts the rotation angle of half-wave plate and quarter-wave plate to adjust the exit energy of incident light until the preset threshold value;Then single-photon detector measures the exit energy of another way of light, adjusts half-wave plate and quarter-wave plate again to keep the exit energy at the level of normal communication.The application can expand the upper limit of the dynamic range of light intensity, realize real-time detection and fine control, and improve the reliability of communication terminal.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of wireless optical communication, in particular to a photon counting communication adaptive receiving device and method. BACKGROUND

[0002] Photon counting communication is a kind of amplitude modulation / direct detection (IM / DD) communication system that uses ultra-high sensitivity photodetectors with single photon detection capability to directly detect and count the time and number of photons arriving, and restores the information of the sending end. The ultra-high energy utilization efficiency makes photon counting communication system very popular in deep space and deep sea wireless communication, which can overcome a large link energy attenuation, and the farthest communication distance can reach 100 million km in deep space and 500 m in deep sea, and there are many application cases.

[0003] Due to the ultra-high detection sensitivity of the single photon detector, it is very easy to enter the saturation state due to the excessive receiving light intensity, and the typical saturation power is 100 pW, and the best communication light intensity is about 5 pW. In actual application scenarios, due to the change of communication distance or channel medium, the power reaching the single photon detector is easy to exceed 1 uW, and the dynamic range reaches 60-80 dB, so it is necessary to strictly control the size of the received power. On the one hand, it is to ensure the continuity of communication, and on the other hand, it is to protect the detector from being damaged. At present, the commonly used control scheme in the photon counting communication system is to passively adjust the receiving end light power by adjusting the communication distance or the light power of the transmitting end, which needs to rely on double-end positioning and information interaction, which increases the application difficulty in deep space and deep sea environment. Therefore, it is urgent to develop a large dynamic range adaptive receiving method for real-time detection and fine control of the receiving light intensity at the receiving end to improve the reliability of the communication terminal.

[0004] In the prior art, a method of using a polarization beam splitter to uniformly divide the incident light into two parts, one part is measured by an optical power meter, and the other part reaches a photodetector for detection. The light intensity adjustment means used in this method is to rotate the angle of the polarizing plate. However, this method is for conventional photodetectors with saturation power in the uW level, and the adjustable light intensity range is 30-40 dB, so it cannot be applied to single photon detection and communication scenarios with higher dynamic range. SUMMARY

[0005] Therefore, the technical problem to be solved by the present application is to overcome the shortcomings of the prior art and provide a photon counting communication adaptive receiving device and method that can expand the upper limit of the light intensity dynamic range, realize real-time detection and fine control, and improve the reliability of the communication terminal.

[0006] To solve the above technical problems, the application provides a photon counting communication adaptive receiving device, which comprises a half-wave plate, a polarization beam splitter, a quarter-wave plate, an angle reflector, a beam splitter, a surface array detector and a single photon detector,

[0007] The incident light passes through the half-wave plate, the polarization beam splitter and the quarter-wave plate in sequence to reach the angle reflector, is reflected by the angle reflector and passes through the quarter-wave plate again to enter the polarization beam splitter, and the light reflected by the polarization beam splitter enters the beam splitter; the beam splitter divides the incident light into two paths, one path of light enters the surface array detector, and the other path of light enters the single photon detector.

[0008] The surface array detector measures the outgoing energy of the one path of light in real time, the outgoing energy of the incident light is adjusted by adjusting the rotation angle of the half-wave plate and the quarter-wave plate, and the outgoing energy of the incident light is reduced to a preset threshold.

[0009] After the outgoing energy of the incident light is reduced to the preset threshold, the single photon detector measures the outgoing energy of the other path of light in real time, the outgoing energy of the incident light is adjusted by adjusting the rotation angle of the half-wave plate and the quarter-wave plate, and the outgoing energy of the incident light is kept at a normal communication level.

[0010] In an embodiment of the application, the device further comprises a first lens and a second lens,

[0011] After the beam splitter divides the incident light into two paths, one path of light enters the surface array detector after passing through the first lens, and the other path of light enters the single photon detector after passing through the second lens.

[0012] In an embodiment of the application, the device further comprises a fiber collimation head and a beam splitting fiber, and the light passing through the second lens enters the single photon detector in sequence after passing through the fiber collimation head and the beam splitting fiber.

[0013] In an embodiment of the application, the device further comprises a calibration laser,

[0014] The outgoing light of the calibration laser reversely converges into the light path of the incident light through the beam splitting fiber, expands in sequence through the fiber collimation head and the second lens, is reflected twice by the beam splitter and the polarization beam splitter in sequence, is reflected in parallel by the angle reflector after passing through the quarter-wave plate, passes through the quarter-wave plate again, is reflected by the polarization beam splitter and the beam splitter again and is focused on the surface array detector through the first lens, the deviation of the spot position on the surface array detector at this time from the factory calibration position is taken as the deviation of the optical axis, and the deviation of the outgoing direction of the incident light is corrected using the deviation of the optical axis.

[0015] In one embodiment of the present application, a telescope is further included for receiving the far-end incident parallel light and projecting the parallel light into the half-wave plate.

[0016] The present application also provides a photon counting communication adaptive receiving method, comprising:

[0017] Receiving the far-end incident light, and sequentially passing the incident light through the half-wave plate, the polarization beam splitter, the quarter-wave plate to the corner reflector, reflecting the incident light by the corner reflector and passing the incident light again through the quarter-wave plate into the polarization beam splitter, and reflecting the light after the incident light is reflected by the polarization beam splitter into the beam splitter; the beam splitter divides the incident light into two paths, one path of light enters the area array detector, and the other path of light enters the single photon detector;

[0018] The area array detector measures the exit energy of the one path of light in real time, adjusts the exit energy of the incident light by adjusting the rotation angle of the half-wave plate and the quarter-wave plate, and until the exit energy of the incident light is reduced to a preset threshold;

[0019] After the exit energy of the incident light is reduced to the preset threshold, the single photon detector measures the exit energy of the other path of light in real time, adjusts the exit energy of the incident light by adjusting the rotation angle of the half-wave plate and the quarter-wave plate, so that the exit energy of the incident light is maintained at a normal communication level.

[0020] In one embodiment of the present application, the exit energy of the incident light is adjusted by adjusting the rotation angle of the half-wave plate and the quarter-wave plate, specifically:

[0021] The azimuth angle of the half-wave plate is represented as θ1, and the azimuth angle of the quarter-wave plate is represented as θ2;

[0022] The expression of the exit energy I of the incident light is:

[0023] I=I0η(cos 2 2θ1)(sin 2 2θ2),

[0024] In the formula, I0 is the incident energy of the incident light, and η is the system efficiency.

[0025] In one embodiment of the present application, the calculation method of the exit energy I of the incident light is specifically:

[0026] The polarization state representation S H of the incident light is calculated respectively, the transmission matrix T of the half-wave plate is calculated, the transmission matrix M PBS-T of the polarization beam splitter is calculated, and the forward transmission matrix T of the quarter-wave plate is calculated, and the transmission matrix M Comer of the corner reflector is calculated., the reverse transmission matrix of a quarter wave plate and the reverse transmission matrix M of a polarizing beam splitter PBS-T , to obtain a final light representation S of the incident light T is:

[0027]

[0028] According to the final light representation S of the incident light T , the exit energy I is obtained.

[0029] In an embodiment of the present application, the S H is expressed as:

[0030] S H = [1 1 0 0] T ,

[0031] The expression of the M is:

[0032]

[0033] The expression of the M PBS-T is:

[0034]

[0035] The expression of the M is:

[0036]

[0037] The expression of the M Comer is:

[0038]

[0039] The expression of the M is:

[0040]

[0041] The expression of the M PBS-T is:

[0042]

[0043] In an embodiment of the present application, according to the final light representation S of the incident light T , the exit energy I is obtained, specifically:

[0044] The S H , M PBS-T , MComer , and M PBS-T The expression of S T is obtained:

[0045] S T = (cos 2 2θ1)(sin 2 2θ2)[1 -1 0 0] T ,

[0046] According to the expression of S T , the relative intensity is (cos 2 2θ1)(sin 2 2θ2), and the expression of the exit energy I of the incident light is obtained in combination with the relative intensity, the incident energy of the incident light and the system efficiency.

[0047] The above technical solutions of the present application have the following advantages compared with the prior art:

[0048] The present application can expand the upper limit of the dynamic range of the incident light intensity to more than 60dB by adjusting the rotation angles of the half-wave plate and the quarter-wave plate to realize dynamic adjustment of the exit energy; and the dynamic range of the incident light intensity can be detected and finely controlled in real time by first coarsely adjusting the exit energy by the area array detector and then finely adjusting the exit energy by the single photon detector, so that the incident light intensity is in the optimal communication interval, and the reliability of the communication terminal is effectively improved. BRIEF DESCRIPTION OF DRAWINGS

[0049] In order to make the content of the present application more easily understood, the present application will be further described in detail below according to specific embodiments of the present application and in combination with the drawings, in which:

[0050] Figure 1 is a structural schematic diagram of the device of the present application,

[0051] Figure 2 is a flowchart of the method of the present application.

[0052] Description of the Drawings: 1, telescope; 2, half-wave plate; 3, polarization beam splitter; 4, quarter-wave plate; 5, corner reflector; 6, beam splitter; 7, first lens; 8, area array detector; 9, second lens; 10, fiber collimating head; 11, beam splitting fiber; 12, single photon detector; 13, calibration laser. DETAILED DESCRIPTION

[0053] The present application will be further described below in combination with the drawings and specific embodiments, so that those skilled in the art can better understand the present application and implement it, but the embodiments are not limiting to the present application.

[0054] Referring to Figure 1 The application discloses a photon counting communication adaptive receiving device, which comprises a half-wave plate 2, a polarization beam splitter 3, a quarter-wave plate 4, an angle reflector 5, a beam splitter 6, a surface array detector 8 and a single photon detector 12. Incident light passes through the half-wave plate 2, the polarization beam splitter 3 and the quarter-wave plate 4 in sequence and reaches the angle reflector 5, is reflected by the angle reflector 5 and passes through the quarter-wave plate 4 again to enter the polarization beam splitter 3, and the light reflected by the polarization beam splitter 3 enters the beam splitter 6. The beam splitter 6 divides the incident light into two paths, one path of light enters the surface array detector 8, and the other path of light enters the single photon detector 12. The surface array detector 8 measures the exit energy of the light in the one path in real time, the exit energy of the incident light is adjusted by adjusting the rotation angle of the half-wave plate 2 and the quarter-wave plate 4, and the exit energy of the incident light is reduced to a preset threshold value. In the embodiment, the preset threshold value is 100 pW, and the surface array detector 8 has a wide field of view, a signal readout circuit and the function of measuring and reading the incident light beam energy. After the exit energy of the incident light is reduced to the preset threshold value, the single photon detector 12 measures the exit energy of the light in the other path in real time, the exit energy of the incident light is adjusted by adjusting the rotation angle of the half-wave plate 2 and the quarter-wave plate 4, and the exit energy of the incident light is kept at a normal communication level. The light passing through the single photon detector 12 completes the communication function. Figure 1 The solid line with an arrow in the middle indicates the light path transmission direction of the incident light.

[0055] The application can expand the upper limit of the dynamic range of the incident light intensity to more than 60 dB by adjusting the rotation angle of the half-wave plate 2 and the quarter-wave plate 4 to realize dynamic adjustment of the exit energy. The dynamic range of the incident light intensity can be detected and controlled in real time and finely by first coarsely adjusting the exit energy by the surface array detector 8 and then finely adjusting the exit energy by the single photon detector 12, so that the incident light intensity is in the optimal communication interval and the reliability of the communication terminal is effectively improved.

[0056] In the embodiment, the photon counting communication adaptive receiving device further comprises a first lens 7 and a second lens 9. After the beam splitter 6 divides the incident light into two paths, one path of light enters the surface array detector 8 after passing through the first lens 7, and the other path of light enters the single photon detector 12 after passing through the second lens 9. The light collecting and constraining effect of the first lens 7 and the second lens 9 facilitates the surface array detector 8 and the single photon detector 12 to collect and measure in real time.

[0057] In the embodiment, the photon counting communication adaptive receiving device further comprises a fiber collimator 10 and a beam splitting fiber 11, and the light passing through the second lens 9 enters the single photon detector 12 through the fiber collimator 10 and the beam splitting fiber 11 in sequence.

[0058] In the embodiment, the photon counting communication adaptive receiving device further comprises a telescope 1, and the telescope 1 is used to receive the parallel light incident from a remote end and project the parallel light into the half-wave plate 2.

[0059] In the embodiment, the photon counting communication adaptive receiving device further comprises a calibration laser 13. The outgoing light of the calibration laser 13 reversely enters the light path of the incident light through the beam splitting fiber 11, is expanded by the fiber collimator 10 and the second lens 9 in sequence, is reflected by the beam splitter 6 and the polarization beam splitter 3 twice in sequence, is parallelly reflected by the corner reflector 5 after passing through the quarter-wave plate 4, passes through the quarter-wave plate 4 again, is reflected by the polarization beam splitter 3 and the beam splitter 6 again, and is focused on the area array detector 8 by the first lens 7, so that the deviation of the spot position on the area array detector 8 at this time from the factory calibration position is taken as the deviation of the optical axis, and the deviation of the outgoing direction of the incident light is corrected using the deviation of the optical axis. Figure 1 The dashed line with an arrow in the middle represents the transmission direction of the outgoing light of the calibration laser 13.

[0060] The application further discloses a photon counting communication adaptive receiving method, which comprises the following steps.

[0061] Step 1: receiving the incident light from a remote end, and making the incident light pass through the half-wave plate 2, the polarization beam splitter 3, the quarter-wave plate 4 and the corner reflector 5 in sequence, reflecting the incident light by the corner reflector 5 and making the incident light pass through the quarter-wave plate 4 again and enter the polarization beam splitter 3, reflecting the incident light by the polarization beam splitter 3, and making the light reflected by the polarization beam splitter 3 enter the beam splitter 6; the beam splitter 6 divides the incident light into two paths, one path of light enters the area array detector 8, and the other path of light enters the single photon detector 12.

[0062] Step 2: adjusting the angles of the half-wave plate 2 and the quarter-wave plate 4 to an initial zero position, and adjusting the phase angle of the incident light multiple times, so that the energy of the incident light is closely related to the rotation angle of the half-wave plate 2 and the quarter-wave plate 4, and the outgoing energy of the incident light can be adjusted by adjusting the rotation angle of the half-wave plate 2 and the quarter-wave plate 4, and the specific adjustment is as follows.

[0063] 1) the polarization state of the incident light is horizontal polarization light, and the polarization state of the incident light is expressed by a Stokes parameter S H is as follows.

[0064] S H = [1 1 0 0] T ,

[0065] 2) The rotatable half-wave plate 2 has an azimuth angle of θ1, and the transmission matrix of the rotatable half-wave plate 2 is:

[0066]

[0067] 3) The incident light is transmitted through the polarizing beam splitter 3, and the forward transmission matrix M of the polarizing beam splitter 3 can be represented as: PBS-T

[0068]

[0069] 4) The transmitted light is transmitted through the rotatable quarter-wave plate 4 with an angle of θ2, and the forward transmission matrix of the quarter-wave plate 4 can be represented as:

[0070]

[0071] 5) The incident light is transmitted through the polarization-maintaining angle reflector 5, and the transmission matrix M of the angle reflector 5 can be represented as: Comer

[0072]

[0073] 6) The light is transmitted through the rotatable quarter-wave plate 4 again, and the angle is exactly opposite to the angle of the normal incidence, which is -θ2, and the reverse transmission matrix of the rotatable quarter-wave plate 4 is:

[0074]

[0075] 7) The incident light is reflected through the polarizing beam splitter 3, and the reverse transmission matrix M of the polarizing beam splitter 3 can be represented as: PBS-T

[0076]

[0077] 8) The light transmitted through the polarizing beam splitter 33 finally can be represented as S: T

[0078]

[0079] The S, H , M PBS-T , M Comer , and M​​​​​​​PBS-T The expression of S is substituted into the expression of T After expansion, the calculation can obtain:

[0080]

[0081] The relative intensity is the first term:

[0082]

[0083] Therefore, the expression of the exit energy I of the incident light is:

[0084] I = I0η (cos 2 2θ1) (sin 2 2θ2),

[0085] Wherein, I0 is the incident light energy, and η is the system efficiency, and in the embodiment, η = 50 %.

[0086] Step 3: The surface array detector 8 measures the exit energy of one light in real time, and the exit energy of the incident light is adjusted by adjusting the rotation angle of the half-wave plate 2 and the quarter-wave plate 4 until the exit energy of the incident light is reduced to a preset threshold.

[0087] Step 4: After the exit energy of the incident light is reduced to the preset threshold, the single photon detector 12 measures the exit energy of another light in real time, and the exit energy of the incident light is adjusted by adjusting the rotation angle of the half-wave plate 2 and the quarter-wave plate 4, so that the exit energy of the incident light is kept at a normal communication level.

[0088] The application can expand the upper limit of the dynamic range of the incident light intensity to more than 60dB by adjusting the rotation angle of the half-wave plate 2 and the quarter-wave plate 4 to realize dynamic adjustment of the exit energy, and can realize real-time detection and fine control of the dynamic range of the incident light intensity by first coarsely adjusting the exit energy by the surface array detector 8 and then finely adjusting the exit energy by the single photon detector 12, so that the incident light intensity is in the best communication interval, and the reliability of the communication terminal is effectively improved.

[0089] Those skilled in the art should understand that the embodiments of the application can be provided as a method, a system or a computer program product. Therefore, the application can adopt a complete hardware embodiment, a complete software embodiment or an embodiment combining software and hardware aspects. Moreover, the application can adopt a computer program product in the form of one or more computer usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer usable program code.

[0090] The computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks. Figure 1 one or more flow or blocks Figure 1 one or more flow or blocks

[0091] These computer program instructions can also be stored in a computer readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer readable memory produce an article of manufacture including instructions which implement the function specified in the flowchart block or blocks. Figure 1 one or more flow or blocks Figure 1 one or more flow or blocks

[0092] The computer program instructions can also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart block or blocks. Figure 1 one or more flow or blocks Figure 1 one or more flow or blocks

[0093] Obviously, the above-described embodiments are only examples and are not intended to limit the present application. Based on the above description, one of ordinary skill in the art can further make other variations and changes to the present application. Here, it is not necessary or possible to enumerate all the embodiments. The obvious changes or variations derived therefrom are still within the scope of the present application.

Claims

1. A photon counting communication adaptive receiving device, characterized by: This includes half-wave plates, polarization beam splitters, quarter-wave plates, corner reflectors, beam splitters, area array detectors, and single-photon detectors. The incident light passes sequentially through the half-wave plate, the polarization beam splitter, and the quarter-wave plate to reach the corner reflector. The incident light is reflected by the corner reflector and passes through the quarter-wave plate again to enter the polarization beam splitter. The incident light after being reflected by the polarization beam splitter enters the beam splitter. The beam splitter splits the incident light into two paths, one of which enters the area array detector and the other enters the single-photon detector. The array detector measures the outgoing energy of one beam in real time, and adjusts the outgoing energy of the incident light by adjusting the rotation angle of the half-wave plate and the quarter-wave plate until the outgoing energy of the incident light is reduced to a preset threshold. After the incident light's outgoing energy decreases to a preset threshold, the single-photon detector measures the outgoing energy of another light source in real time. By adjusting the rotation angles of the half-wave plate and the quarter-wave plate, the outgoing energy of the incident light is adjusted so that it remains at the level required for normal communication.

2. The photon counting communication adaptive receiving device according to claim 1, characterized in that: It also includes a first lens and a second lens. The beam splitter splits the incident light into two paths. One path passes through the first lens and enters the area array detector, while the other path passes through the second lens and enters the single-photon detector.

3. The photon counting communication adaptive receiving apparatus according to claim 2, characterized in that: It also includes an optical fiber collimator and a beam splitter. The light passing through the second lens passes sequentially through the optical fiber collimator and the beam splitter before entering the single-photon detector.

4. The photon counting communication adaptive receiving apparatus according to claim 3, characterized in that: It also includes calibrating the laser. The emitted light from the calibration laser is reversed by the beam splitter and merged into the incident light's optical path. It then passes through the fiber collimator and the second lens for beam expansion, is reflected twice by the beam splitter and the polarization beam splitter, passes through the quarter-wave plate, is reflected parallel by the corner reflector, passes through the quarter-wave plate again, is reflected once more by the polarization beam splitter and the beam splitter, and is focused onto the area array detector by the first lens. The deviation of the spot position on the area array detector from the factory calibration position is taken as the optical axis deviation, and the optical axis deviation is used to correct the emission direction of the incident light.

5. The photon counting communication adaptive receiving device according to any of claims 1 to 4, characterized in that: It also includes a telescope for receiving parallel light incident from a distance and directing the parallel light onto the half-wave plate.

6. A photon counting communication adaptive receiving method, characterized in that, include: The system receives incident light from a distant point and passes it sequentially through a half-wave plate, a polarizing beam splitter, and a quarter-wave plate to a corner reflector. The incident light is reflected by the corner reflector and passes through the quarter-wave plate again to enter the polarizing beam splitter. The incident light, after being reflected by the polarizing beam splitter, enters the beam splitter. The beam splitter splits the incident light into two paths: one path enters the area array detector, and the other path enters the single-photon detector. The array detector measures the outgoing energy of one beam in real time, and adjusts the outgoing energy of the incident light by adjusting the rotation angle of the half-wave plate and the quarter-wave plate until the outgoing energy of the incident light is reduced to a preset threshold. After the exit energy of the incident light decreases to a preset threshold, the single-photon detector measures the exit energy of another light in real time, adjusts the exit energy of the incident light by adjusting the rotation angles of the half-wave plate and the quarter-wave plate, so that the exit energy of the incident light is kept at a level of normal communication.

7. The method of claim 6, wherein: The exit energy of the incident light is adjusted by adjusting the rotation angles of the half-wave plate and the quarter-wave plate, specifically: The azimuth angle of the half-wave plate is denoted as θ1, and the azimuth angle of the quarter-wave plate is denoted as θ2. The expression of the exit energy I of the incident light is: I = I0η(cos 2 2θ1)(sin 2 2θ2), In the formula, I0 is the incident energy of the incident light, and η is the system efficiency.

8. The method of claim 7, wherein: The calculation method of the exit energy I of the incident light is specifically: The polarization state of the incident light is calculated separately, denoted S H The transmission matrix of the half-wave plate The transmission matrix M of the polarizing beam splitter PBS-T The forward transmission matrix of the quarter-wave plate The transmission matrix M of the corner reflector Comer The backward transmission matrix of the quarter-wave plate The backward transmission matrix M of the polarizing beam splitter PBS-T The final light representation S of the incident light is obtained by T : According to the final light representation S of the incident light T The exit energy I is obtained.

9. The method of claim 8, wherein: The S H The expression is: S H ＝[1 1 0 0] T ， The The expression is: The M PBS-T The expression is: The Expression is: The M Comer The expression is: The Expression is: The M PBS-T expression is:

10. The method of claim 9, wherein: According to the final light representation S of the incident light T The exit energy I is obtained, in particular: S H , M PBS-T , M Comer , and M PBS-T ′ into the final light representation S T of the incident light gives: S T = (cos 2 2θ1)(sin 2 2θ2)[1-1 0 0] T , According to S T The relative intensity is (cos 2 2θ1)(sin 2 2θ2), and the expression of the exit energy I of the incident light is obtained by combining the relative intensity, the incident energy of the incident light, and the system efficiency.

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