An optical spectroscopy system, comprising: an attachment unit attached to a specific region of a subject and configured to, based on firmware, emit light to the specific region using a reference clock and collect light coming through the specific region to measure a biosignal of the subject; and a monitoring unit connected to the attachment unit through a wired or wireless network and configured to control an intensity of the light emitted by the attachment unit, wherein the attachment unit comprises: a) a light transmission unit configured to emit light to the specific region using at least one of a laser and a light emitting diode, said light transmission unit including a plurality of light transmission modules configured to emit light of different channels modulated with time-divided spread spectrum codes (TDSSC), wherein said TDSSC codes are Walsh codes; b) a light reception unit configured to detect light coming through the specific region; and c) a main processing unit configured to control the light transmission unit and the light reception unit based on the firmware, and receive output data of the light reception unit, wherein the firmware is configured to demodulate data for the light received by the light reception unit based on the time-divided spread spectrum codes to separate the output data into the optical data of each channel, characterized in that the light transmission unit is further configured to a) decrease or increase an on/off duty ratio of the light emission by a predetermined ratio while increasing or decreasing, respectively, the intensity of light emission by said predetermined ratio, thereby maintaining constant the injected energy level of the emitted light, and b) start the Walsh codes simultaneously and turn them off simultaneously, thereby maintaining orthogonality between the codes. an attachment unit attached to a specific region of a subject and configured to, based on firmware, emit light to the specific region using a reference clock and collect light coming through the specific region to measure a biosignal of the subject; and a monitoring unit connected to the attachment unit through a wired or wireless network and configured to control an intensity of the light emitted by the attachment unit, wherein the attachment unit comprises: a) a light transmission unit configured to emit light to the specific region using at least one of a laser and a light emitting diode, said light transmission unit including a plurality of light transmission modules configured to emit light of different channels modulated with time-divided spread spectrum codes (TDSSC), wherein said TDSSC codes are Walsh codes; b) a light reception unit configured to detect light coming through the specific region; and c) a main processing unit configured to control the light transmission unit and the light reception unit based on the firmware, and receive output data of the light reception unit, wherein the firmware is configured to demodulate data for the light received by the light reception unit based on the time-divided spread spectrum codes to separate the output data into the optical data of each channel, a) a light transmission unit configured to emit light to the specific region using at least one of a laser and a light emitting diode, said light transmission unit including a plurality of light transmission modules configured to emit light of different channels modulated with time-divided spread spectrum codes (TDSSC), wherein said TDSSC codes are Walsh codes; b) a light reception unit configured to detect light coming through the specific region; and c) a main processing unit configured to control the light transmission unit and the light reception unit based on the firmware, and receive output data of the light reception unit, a) decrease or increase an on/off duty ratio of the light emission by a predetermined ratio while increasing or decreasing, respectively, the intensity of light emission by said predetermined ratio, thereby maintaining constant the injected energy level of the emitted light, and b) start the Walsh codes simultaneously and turn them off simultaneously, thereby maintaining orthogonality between the codes.