3.1.

Electrical Connection Among Modules

MPU and programmable analog/digital circuits can reconstruct the function of the system. Command-based operation of each module is realized. Optical modules are schematically shown in Fig. 5. All analog circuits are built-in. HV represent a high-voltage supplier. These optical modules are connected via an ${\mathrm{I}}^2\mathrm{C}$ bus, a daisy-chain communication protocol. Since the base and data clocks are shared, optical emission and detection timings are synchronized. Power supply for LED light sources, power supply for electronic circuits, ground, data clock, base clock (BC), and ${\mathrm{I}}^2\mathrm{C}$ bus (serial clock line and serial data line) are shared among all modules.

Fig. 5

Schematic of optical modules. All analog circuits are built-in. MPU and HV represent microprocessing unit and high voltage supplier, respectively.

3.2.

Basic Structure of LED/APD Module and Control Unit

LED/APD modules and control unit are connected to each other in a daisy-chain configuration using an ${\mathrm{I}}^2\mathrm{C}$ bus. Up to 128 slave addresses (i.e., modules) can be assigned in our system. These modules can be adequately applied to many kinds of measurements from one-channel to multichannel brain-activity monitoring. They share information using a memory buffer in each LED/APD module that is read or written by the control unit. The memory buffer includes setting parameters and measurement data that are read and written by an MPU. The control unit sends data to a personal computer (PC) via wired/wireless local area network and hemoglobin (Hb) calculation is performed by software at the PC. The molar extinction coefficients (as a function of wavelength) can be set for spectroscopy analysis (i.e., modified Beer–Lambert law⁴⁴), and analog-to-digital (A/D) converted optical intensity signal at each light source can be saved in a text file as well. Figure 6 schematically shows the communication network connections between LED/APD modules and a control unit.

Fig. 6

Schematic of communication network connections between LED/APD modules and control unit. Control unit plays a role of a master and SoCs play roles of slaves in the ${\mathrm{I}}^2\mathrm{C}$ bus.

3.3.

Frequency Generation

LED and APD modules generate frequencies for modulating/detecting optical signals by dividing the BC, 163,840 Hz, from the control unit. The number of divisions can be set to any integer value. By using programmable frequency divider components and flip-flop circuits, over six frequencies of a common divisor can be generated. The frequency phases are easy to synchronize because the frequencies share the same BC.

A schematic of generation of clocks at LED/APD modules for analog-to-digital (A/D) conversion (ADC), frequency modulation, and data sampling is shown in Fig. 7.

Fig. 7

Generation of clocks at LED/APD modules for A/D conversion, frequency modulation, and data sampling.

3.4.

Time-Division and Frequency-Division Lock-in Detection

Up to four positions of light sources are discriminated by timing of optical signals, and two wavelengths are discriminated by two frequencies of modulation. Frequencies of the first and second nearest sources emitted at the same time are set differently to avoid crosstalk. Since each APD module can detect two signals (i.e., wavelengths) at the same time, more than two signals should be separately detected by time divisions (details are stated in Sec. 3.7). Table 2 shows a relationship between probe arrangement and conditions for time-division lock-in detection [minimum number of time divisions (T) and number of frequency divisions (F)]. The probe arrangements are assumed as 30-mm lattice arrangements with light source on top left position. In all probe arrangements, minimum numbers of frequency divisions (F) are two, with reference to a practical criteria mentioned in Sec. 3.5, but they can be set to more than two with the same or better signal quality.

Table 2

Relationship between probe arrangement and conditions for time-division lock-in detection.

Atsumori et al.²⁴ reported time-divided digital lock-in detection, which enables cross-talk-free measurements because no multiple light sources are simultaneously emitted. In this technology, multiple lights modulated in different frequencies are simultaneously irradiated on the scalp to detect and separate the multiple lights emitted from different light sources (frequency lock-in detection method). Time-divided digital lock-in detection is useful for biological optical measurements, such as simultaneous multiple-point measurements, but these frequencies must be set sufficiently far apart or the light sources should be placed sufficiently far apart to prevent interference between light sources.

3.5.

Possible Probe Arrangements

An example $5\times 5$ probe arrangement using four time divisions and six frequencies is shown in Fig. 8. Ideally, there should be as many frequencies as light sources used simultaneously, but the same frequencies can be used simultaneously at distant positions from each other. In the case of Fig. 8, detector No. 1 (D1) detects signals of frequencies 3 and 4 (f3 and f4) at division 3 (number in light-source circle) from both sources No. 1 (S1) and No. 13 (S13). However, S-D distances for S1-D1 and S13-D1 combinations are 30 and 150 mm, respectively, and the latter signal is much weaker than the former one (which can be considered to be zero).

Fig. 8

Example $5\times 5$ probe arrangement using 4 time divisions and 6 frequencies. White and black circles indicate positions for light sources (LEDs, S1–13) and detectors (APDs, D1–12). The numbers in a white circle indicate light-emitting timings. The numbers with “f” above light sources are frequency indices.

When frequencies and timing can be changed individually, a $2\times 12$ probe arrangement using only three time divisions and four frequencies, for example, can be configured, as shown in Fig. 9.

Fig. 9

Example $2\times 12$ probe arrangement using 3 time divisions and 4 frequencies. White and black circles indicate positions for light sources (LEDs) and detectors (APDs), respectively.

The rearrangeable concept is useful for optimal measurements depending on the purpose (signal-to-noise ratio, number of measurement positions, etc.). The distance between LED and APD modules is preferably set to 30 mm because it is reported that a cerebral signal can be detected on a human scalp with sources and detectors 30 mm apart.⁴⁵

When the distance between LED and APD modules on a human scalp is expanded from 30 to 50 mm, the detected optical intensity exponentially reduces to one-hundredth (1%).⁴⁶ In other words, the intensity reduces to one-tenth at each 10 mm. For one APD module, the intensity of the signal from an LED module 65 mm away is less than one-thousandth of that from an LED module 30 mm away, which can be practically ignored. In this way, for one APD, there should be no more than one LED module modulated with the same frequency emitting at the same time within a 65 mm distance. Therefore, any 30-mm lattice arrangement can be configured by using only four time divisions and two frequencies (Table 2).

APD modules can be added for short-distance signal acquisitions. APD modules do not affect other channels because they do not emit light, adding APD modules can therefore be performed by only setting their frequencies and timings corresponding to light signals they should detect. The multidistance measurement with a high-density probe arrangement thus can be performed by just adding and setting APD modules.

3.6.

Light-Emitting Diode Module

A block diagram of an LED module is shown in Fig. 10. Frequency for amplitude modulation is generated by dividing the BC. The optical output is controlled by digital-to-analog converters for power level control and transistors (TRA) for switching. Output level, timing signal, and frequency selection are controlled by parameters stored in an ${\mathrm{I}}^2\mathrm{C}$ buffer in an SoC static random-access memory (SRAM). Frequency is selected by digital multiplexers (MUX). The monitor photodiode (mPD) output is amplified at a TIA and PGA, the gain of which can be set by an MPU. Detected signals are analog-to-digital converted for each wavelength $\lambda$ and stored at the BC rate in each memory field automatically by a MPU process. The data are averaged among specific numbers and stored as power output data, which can be used for APC.

Fig. 10

Block diagram for LED module.

3.7.

Avalanche Photodiode Module

A block diagram of an APD module is shown in Fig. 11. Four-channel data are simultaneously obtained the most by dividing detection times (1–4). An APD is used for a detector and the photocurrent signal is amplified at a TIA and one fixed amplifier and two PGAs. Six or more frequencies are generated by dividing the BC, and any frequency can be technically set for each time division at each wavelength $\lambda$ ($f1-f8$). Frequency is selected at each detection timing by digital MUX. The reference signal of modulation frequencies and optical signal amplified by PGAs is multiplied at an analog mixer in a hardware lock-in process. The processed signals are A/D converted at the BC, and all digitalized data are stored in SRAM via dynamic memory access in the SoC. The MPU processes averaging the data and the lock-in signal is obtained at each timing and each wavelength $\lambda$. The averaged data are recorded in the ${\mathrm{I}}^2\mathrm{C}$ buffer and accessed (acquired) by the control unit via the ${\mathrm{I}}^2\mathrm{C}$ bus. A high voltage supplier (HV) for the APD is also implemented in the module. For high voltage control, 3.3-V supplied voltage is boosted from 80 to 90-V reverse voltage by a DC-to-DC converter.

Fig. 11

Block diagram for APD module.

3.8.

Automatic Power and Gain Control

The LED module has an APC function. This is an MPU-processed function of monitoring the power of an LED and making it constant by controlling the driving current for the LED. The signals of two wavelengths are separately detected by analog lock-in detection because two wavelengths of light are simultaneously detected by a single mPD. The driving currents for two wavelengths can be independently controlled. A proportional-integral-derivative (PID) controller is implemented in MPU processing, and the parameters are stored in the internal memory. The optical output power can be changed on a PC through the control unit.

A test was conducted to demonstrate the performance of APC. The temperature of the LED module was changed by Peltier controller, and power output of mPD was monitored. Output power change (mPD based) of the LED depending on temperature when APC with PID was used or not is shown in Fig. 12. The output power change of the LED was controlled within 0.1% (standard deviation: 0.05%).

Fig. 12

Output power change of LED depending on temperature when APC with PID is used or not.

The APD modules also perform AGC of optical detection. Each APD module sets each gain of PGA to obtain the suitable signal amplitude. Such autonomous AGC can be simultaneously performed at all channels because each module has an MPU in an SoC.

3.9.

Fitting Indicator of Headset

Because each LED/APD module has an MPU in an SoC that autonomously controls optical power and detector gain, each module has information of the fitting status of a headset (holder of modules placed on the head). Using this advantage, the module has a color LED indicator on the module case so that the color of the LED indicator tells users the fitting status of the headset, which contributes to shortening the preparation time of the measurement. No additional electric wire is necessary between each module and control unit.